Remove references to closed mailing list

http://robots.thoughtbot.com/moving-open-source-project-mailing-lists-to-stack-overflow/

README.md

@@ -714,11 +714,11 @@ If you'd like to contribute a feature or bugfix: Thanks! To make sure your
 fix/feature has a high chance of being included, please read the following
 fix/feature has a high chance of being included, please read the following
 guidelines:
 guidelines:
 
 
-1. Ask on the [mailing list](http://groups.google.com/group/paperclip-plugin), or
-   post a new [GitHub Issue](http://github.com/thoughtbot/paperclip/issues).
+1. Post a [pull request](https://github.com/thoughtbot/paperclip/compare/).
 2. Make sure there are tests! We will not accept any patch that is not tested.
 2. Make sure there are tests! We will not accept any patch that is not tested.
    It's a rare time when explicit tests aren't needed. If you have questions
    It's a rare time when explicit tests aren't needed. If you have questions
-   about writing tests for paperclip, please ask the mailing list.
+   about writing tests for paperclip, please open a
+   [GitHub issue](https://github.com/thoughtbot/paperclip/issues/new).
 
 
 Please see `CONTRIBUTING.md` for more details on contributing and running test.
 Please see `CONTRIBUTING.md` for more details on contributing and running test.