Add documentation for creating a release

Add steps for creating a release to CONTRIBUTING.md.

CONTRIBUTING.md

@@ -53,3 +53,20 @@ branches.
 
 Please see our [git style guide](https://github.com/alphagov/styleguides/blob/master/git.md)
 which describes how we prefer git history and commit messages to read.
+
+## To release a new version
+
+Update [CHANGELOG.md](https://github.com/alphagov/govuk_prototype_kit/blob/master/CHANGELOG.md) to summarise the changes made since the last release.
+
+To see the commits to be summarised in the changelog since the last release, [compare the latest-release branch with master](https://github.com/alphagov/govuk_prototype_kit/compare/latest-release...master).
+
+Propose a new version number in [VERSION.txt](https://github.com/alphagov/govuk_prototype_kit/blob/master/VERSION.txt) and update line 4 in [package.json](https://github.com/alphagov/govuk_prototype_kit/blob/master/package.json#L4) with the new version number.
+
+Open a new pull request with a single commit including the above changes.
+
+[Here is an example for v6.1.0](https://github.com/alphagov/govuk_prototype_kit/commit/53e36d79a994ce3649b53f4008370cd75068c27c).
+
+Once merged into master a new version will be built.
+
+
+