Add documentation for GitHub PR comparison feature

Co-authored-by: letmaik <530988+letmaik@users.noreply.github.com>

Author: Copilot

README.md

@@ -12,6 +12,8 @@ In bigger projects with many files it also provides **context**, it gives you a
 
 - Working tree comparison against any chosen branch, tag, or commit
 
+- **Compare GitHub Pull Requests** - Enter a PR URL to quickly view and compare PR changes without leaving VS Code
+
 - Switch between tree and list view
 
 - Compare in merge or full mode
@@ -34,6 +36,24 @@ By default, the tree view is located in its own container accessible from the ac
 
 <img src="screenshots/move-view.gif" alt="Moving of Git Tree Compare view between containers" width="256" />
 
+## Compare GitHub Pull Requests
+
+You can quickly view GitHub PR changes directly in VS Code using the **Compare GitHub Pull Request** command:
+
+1. Click the "..." menu button in the Git Tree Compare view title bar
+2. Select **Compare GitHub Pull Request...**
+3. Enter the GitHub PR URL (e.g., `https://github.com/owner/repo/pull/123`)
+4. Authenticate with GitHub if prompted (uses VS Code's built-in GitHub authentication)
+5. The extension will:
+   - Fetch the PR's head commit
+   - Checkout the PR branch
+   - Compare it against the PR's base branch
+   - Display all changes in the tree view
+
+This feature works with both PRs from the same repository and PRs from forks. If you have uncommitted changes, you'll be prompted to confirm before checking out the PR.
+
+**Note:** This will checkout the PR in detached HEAD state. You can switch back to your previous branch using the Source Control view or by running `git checkout <branch-name>` in the terminal.
+
 ## Settings
 
 `gitTreeCompare.diffMode` Determines how the comparison is performed, either by computing a merge base commit first and then comparing against that (equivalent to pull request diffs, default), or by comparing directly to the given base (useful to see the exact diff).