Update README with project initialization instructions (#1772)

* Update README with project initialization instructions

Added instructions for creating a new project and initializing in an existing project.

* Update README.md with alternative one-time usage command for existing projects

Co-authored-by: Copilot <175728472+Copilot@users.noreply.github.com>

* Added --ai option to prevent interactive AI selection

Co-authored-by: Copilot <175728472+Copilot@users.noreply.github.com>

---------

Co-authored-by: Copilot <175728472+Copilot@users.noreply.github.com>

Author: lubatti

README.md

@@ -80,7 +80,13 @@ uv tool install specify-cli --force --from git+https://github.com/github/spec-ki
 Run directly without installing:
 
 ```bash
+# Create new project
 uvx --from git+https://github.com/github/spec-kit.git specify init <PROJECT_NAME>
+
+# Or initialize in existing project
+uvx --from git+https://github.com/github/spec-kit.git specify init . --ai claude
+# or
+uvx --from git+https://github.com/github/spec-kit.git specify init --here --ai claude
 ```
 
 **Benefits of persistent installation:**