PP-2338 Add contribution statement

As per other Pay repros

CONTRIBUTING.md

@@ -0,0 +1,11 @@
+# GOV.UK Pay contributing guide
+
+This guide covers the basics of how to contribute to the GOV.UK Pay project.
+
+## Pull requests
+The team's pull request checklist can be found [here](https://github.com/alphagov/pay-team-manual/blob/master/docs/development-processes/pull-request-checklist.md)
+
+## Contributions from beyond the team
+If you have an idea to share or a feature to request to raise please contact the GOV.UK Pay team [email protected]. 
+
+If this is a security issue please do not submit a pull request or raise a GitHub issue, instead, please read the disclosure process [here](/README.md#responsible-disclosure).