Add mailing list to readme

Author: amacneil

README.md

@@ -390,6 +390,10 @@ If you are having general issues with Omnipay, we suggest posting on
 [Stack Overflow](http://stackoverflow.com/). Be sure to add the
 [omnipay tag](http://stackoverflow.com/questions/tagged/omnipay) so it can be easily found.
 
+If you want to keep up to date with release anouncements, discuss ideas for the project,
+or ask more detailed questions, there is also a [mailing list](https://groups.google.com/forum/#!forum/omnipay) which
+you can subscribe to.
+
 If you believe you have found a bug, please report it using the [GitHub issue tracker](https://github.com/adrianmacneil/omnipay/issues),
 or better yet, fork the library and submit a pull request.
 
@@ -407,7 +411,7 @@ or better yet, fork the library and submit a pull request.
 ## Feedback
 
 **Please provide feedback!** We want to make this library useful in as many projects as possible.
-Please raise a Github issue, and point out what you do and don't like, or fork the project and make
-suggestions. **No issue is too small.**
+Please head on over to the [mailing list](https://groups.google.com/forum/#!forum/omnipay)
+and point out what you do and don't like, or fork the project and make suggestions. **No issue is too small.**
 
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