Add GitHub Updater note to readme.

Author: Matthew Mirus

README.md

@@ -14,3 +14,5 @@ Example:
 
 RegEx Pattern: ^1[0-9]{1}$
 Validation Message: Please enter a number between 10-19 (inclusive).
+
+Note: this plugin optionally can be updated using the [Github Updater plugin](https://github.com/afragen/github-updater).