Added a contribution section.

Author: mlh758

README.markdown

@@ -7,3 +7,7 @@ MultiSelect progessively enhances an ordinary multiple select control into elega
 # License
 
 MultiSelect is dual-licensed under the [GPL 2 license](https://github.com/ehynds/jquery-ui-multiselect-widget/blob/master/GPL-LICENSE) and the [MIT license](https://github.com/ehynds/jquery-ui-multiselect-widget/blob/master/MIT-LICENSE).
+
+# Contributing
+
+When submitting a pull request, please describe the change you are making - preferably with a use case. Unit tests are now (14 March 2016) required for the pull to be merged.