Add instructions for using the repo as an antigen plugin

This should also work with Prezto, but I've only tested with antigen
since that's what I use.

README.md

@@ -38,6 +38,16 @@ Usage
 
         rm -f ~/.zcompdump; compinit
 
+
+#### Using antigen
+Add
+```bash
+antigen bundle zsh-lovers/zsh-completions
+```
+
+to your `.zshrc`. This will automatically clone the repository and add zsh-completions/src to your `$fpath`. You may have to rebuild zcompdump as listed above under Manual Installation.
+
+
 Contributing
 ------------