gpg-agent: add README (#7313)

plugins/gpg-agent/README.md

+# gpg-agent
+
+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:
+```
+plugins=(... gpg-agent)
+```