Merge pull request #6174 from NHSDigital/contributing

Add some contributing text about code reviews

Author: thomasleese

CONTRIBUTING.md

@@ -0,0 +1,11 @@
+# Contributing
+
+## Code reviews
+
+All pull requests should be reviewed by another developer on the team before
+they can be merged in.
+
+Don't be afraid to use "Request changes" when suggesting changes to a pull
+request. This helps to make it easier to filter in lists of pull requests
+which are still in need of a review, and the person who raised the pull
+request can still dismiss the review if they disagree.