Update README to link to GPC

Author: jamesfredley

README.md

@@ -1,3 +1,7 @@
+# Important!
+
+Grails Mail plugin is now maintained under the [Grails Plugin Collective](https://github.com/gpc/grails-mail)
+
 # Grails Mail Plugin
 
 [![Maven Central](https://img.shields.io/maven-central/v/org.grails.plugins/mail.svg?label=Maven%20Central)](https://central.sonatype.com/artifact/org.grails.plugins/mail)
@@ -40,4 +44,4 @@ Issues can be raised via [GitHub Issues](https://github.com/grails/grails-mail/i
 
 Pull requests are the preferred method for submitting contributions. Please open an issue via that issue tracker link above and create an issue describing what your contribution addresses.
 
-If you are contributing documentation, raising an issue is not necessary.
+If you are contributing documentation, raising an issue is not necessary.