git-remote-branch: add README (#8259)

plugins/git-remote-branch/README.md

+# git-remote-branch plugin
+
+This plugin adds completion for [`grb`](https://github.com/webmat/git_remote_branch),
+or `git_remote_branch`.
+
+To use it, add `git-remote-branch` to the plugins array of your `.zshrc` file:
+```
+plugins=(... git-remote-branch)
+```
+
+## Deprecation
+
+[git_remote_branch was archived in 2018](https://github.com/webmat/git_remote_branch#archived),
+meaning it's not actively maintained anymore. Use at your own risk.