add link to documentation

plugins/gpg-agent/README.md

 # gpg-agent
 
-Enable gpg-agent if it is not running.
+Enables [GPG's gpg-agent](https://www.gnupg.org/documentation/manuals/gnupg/) if it is not running.
 
 To use it, add gpg-agent to the plugins array of your zshrc file:
 ```