chore(github): add links to supported tools (#10057)

plugins/github/README.md

@@ -2,8 +2,8 @@
 
 This plugin supports working with GitHub from the command line. It provides a few things:
 
-* Sets up the `hub` wrapper and completions for the `git` command if you have `hub` installed.
-* Completion for the `github` Ruby gem.
+* Sets up the `hub` wrapper and completions for the `git` command if you have [`hub`](https://github.com/github/hub) installed.
+* Completion for the [`github` Ruby gem](https://github.com/defunkt/github-gem).
 * Convenience functions for working with repos and URLs.
 
 ### Functions