docs: separate install information from README

In an effort to make our README more concise, move GCM install
instructions into a new install.md file. Additionally, add a section
detailing how to install GCM as a dotnet tool.

Author: ldennington

README.md

@@ -19,6 +19,10 @@ Git Credential Manager (GCM) replaces the .NET Framework-based
 [Git Credential Manager for Mac and Linux][gcm-for-mac-and-linux] (Java GCM),
 providing a consistent authentication experience across all platforms.
 
+## Install
+
+See [installation instructions][install] for your operating system.
+
 ## Current status
 
 Git Credential Manager is currently available for Windows, macOS, and Linux\*.
@@ -53,186 +57,10 @@ fully-supported.
 - Fedora/CentOS/RHEL
 - Alpine
 
-## Download and Install
-
-### macOS Homebrew
-
-The preferred installation mechanism is using Homebrew; we offer a Cask in our
-custom Tap.
-
-To install, run the following:
-
-```shell
-brew tap microsoft/git
-brew install --cask git-credential-manager-core
-```
-
-After installing you can stay up-to-date with new releases by running:
-
-```shell
-brew upgrade git-credential-manager-core
-```
-
-#### Git Credential Manager for Mac and Linux (Java-based GCM)
-
-If you have an existing installation of the 'Java GCM' on macOS and you have
-installed this using Homebrew, this installation will be unlinked
-(`brew unlink git-credential-manager`) when GCM is installed.
-
-#### Uninstall
-
-To uninstall, run the following:
-
-```shell
-brew uninstall --cask git-credential-manager-core
-```
-
----
-
-### macOS Package
-
-We also provide a [.pkg installer][latest-release] with each release. To install,
-double-click the installation package and follow the instructions presented.
-
-#### Uninstall
-
-To uninstall, run the following:
-
-```shell
-sudo /usr/local/share/gcm-core/uninstall.sh
-```
-
----
-
-<!-- this explicit anchor should stay stable so that external docs can link here -->
-<!-- markdownlint-disable-next-line no-inline-html -->
-<a name="linux-install-instructions"></a>
-
-### Linux
-
-#### Ubuntu/Debian distributions
-
-Download the latest [.deb package][latest-release], and run the following:
-
-```shell
-sudo dpkg -i <path-to-package>
-git-credential-manager configure
-```
-
-**Note:** Although packages were previously offered on certain
-[Microsoft Ubuntu package feeds][ms-package-repos],
-GCM no longer publishes to these repositories. Please install the
-Debian package using the above instructions instead.
-
-To uninstall:
-
-```shell
-git-credential-manager unconfigure
-sudo dpkg -r gcmcore
-```
-
-#### Other distributions
-
-##### Option 1: Tarball
-
-Download the latest [tarball][latest-release], and run the following:
-
-```shell
-tar -xvf <path-to-tarball> -C /usr/local/bin
-git-credential-manager configure
-```
-
-To uninstall:
-
-```shell
-git-credential-manager unconfigure
-rm $(command -v git-credential-manager)
-```
-
-#### Option 2: Install from source helper script
-
-1. Ensure `curl` is installed:
-
-   ```shell
-   curl --version
-   ```
-
-   If `curl` is not installed, please use your distribution's package manager
-   to install it.
-
-1. Download and run the script:
-
-   ```shell
-   curl -LO https://aka.ms/gcm/linux-install-source.sh &&
-   sh ./linux-install-source.sh &&
-   git-credential-manager-core configure
-   ```
-
-   **Note:** You will be prompted to enter your credentials so that the script
-   can download GCM's dependencies using your distribution's package
-   manager.
-
-To uninstall:
-
-[Follow these instructions][linux-uninstall] for your distribution.
-
-**Note:** all Linux distributions
-[require additional configuration][gcm-credstores] to use GCM.
-
----
-
-### Windows
-
-GCM is included with [Git for Windows][git-for-windows], and the latest version
-is included in each new Git for Windows release. This is the preferred way to
-install GCM on Windows. During installation you will be asked to select a
-credential helper, with GCM being set as the default.
-
-![image][git-for-windows-screenshot]
-
-#### Standalone installation
-
-You can also download the [latest installer][latest-release] for Windows to
-install GCM standalone.
-
-**:warning: Important :warning:**
-
-Installing GCM as a standalone package on Windows will forcibly override the
-version of GCM that is bundled with Git for Windows, **even if the version
-bundled with Git for Windows is a later version**.
-
-There are two flavors of standalone installation on Windows:
-
-- User (preferred) (`gcmuser-win*`):
-
-  Does not require administrator rights. Will install only for the current user
-  and updates only the current user's Git configuration.
-
-- System (`gcm-win*`):
-
-  Requires administrator rights. Will install for all users on the system and
-  update the system-wide Git configuration.
-
-To install, double-click the desired installation package and follow the
-instructions presented.
-
-#### Uninstall (Windows 10)
-
-To uninstall, open the Settings app and navigate to the Apps section. Select
-"Git Credential Manager" and click "Uninstall".
-
-#### Uninstall (Windows 7-8.1)
-
-To uninstall, open Control Panel and navigate to the Programs and Features
-screen. Select "Git Credential Manager" and click "Remove".
-
-#### Windows Subsystem for Linux (WSL)
-
-Git Credential Manager can be used with the [Windows Subsystem for Linux
-(WSL)][ms-wsl] to enable secure authentication of your remote Git
-repositories from inside of WSL.
+## Install
 
-[Please see the GCM on WSL docs][gcm-wsl] for more information.
+See [install.md](install) for installation instructions for your operating
+system.
 
 ## Supported Git versions
 
@@ -328,15 +156,10 @@ When using GitHub logos, please be sure to follow the
 [gcm-net-config]: docs/netconfig.md
 [gcm-usage]: docs/usage.md
 [gcm-windows-broker]: docs/windows-broker.md
-[gcm-wsl]: docs/wsl.md
-[git-for-windows]: https://gitforwindows.org/
-[git-for-windows-screenshot]: https://user-images.githubusercontent.com/5658207/140082529-1ac133c1-0922-4a24-af03-067e27b3988b.png
 [git-tools-credential-storage]: https://git-scm.com/book/en/v2/Git-Tools-Credential-Storage
 [github]: https://github.com
 [github-ssh]: https://help.github.com/en/articles/connecting-to-github-with-ssh
 [github-logos]: https://github.com/logos
-[latest-release]: https://github.com/GitCredentialManager/git-credential-manager/releases/latest
-[linux-uninstall]: docs/linux-fromsrc-uninstall.md
+[install]: docs/install.md
 [ms-package-repos]: https://packages.microsoft.com/repos/
-[ms-wsl]: https://aka.ms/wsl#
 [workflow-status]: https://github.com/GitCredentialManager/git-credential-manager/actions/workflows/continuous-integration.yml