Update readme to be slightly more informative

Author: spalmurray

README.md

@@ -4,4 +4,29 @@
 
 The Sentry Prevent CLI is responsible for uploading code coverage and test results to Sentry. It can be used directly or indirectly through our [Sentry Prevent GitHub Action](https://github.com/getsentry/prevent-action).
 
-This Repo and README are still a work in progress.
+## What happened to the Codecov CLI?
+
+The Codecov CLI is still around! We have moved the Codecov CLI's codebase into the `codecov-cli` sub-project in this repository because the Sentry Prevent CLI uses it as a backend and we wanted to keep the projects together in the `getsentry` organization. The Codecov CLI is still being maintained, improved, and published - just from this repository. See the sub-project's [README](/codecov-cli/README.md) for Codecov CLI specific information, the rest of this document will be about the Prevent CLI.
+
+- [Installing](#installing)
+- [Usage](#usage)
+- [Contributions](#contributions)
+- [Releases](#releases)
+
+# Installing
+
+As of today, you are not able to install the Sentry Prevent CLI. We are planning to release it on the PyPI and for direct download in the near future.
+
+If you're determined, you can build the CLI yourself from source by running `uv build` in the `prevent-cli` directory.
+
+# Usage
+
+If the installation is successful, running `sentry-prevent-cli --help` will output the available commands along with the different options that can be used with them.
+
+# Contributions
+
+Todo
+
+# Releases
+
+Todo