Working draft release notes

https://spec.graphql.org/ has release notes for everything except the working draft. This makes it difficult to know what changes are going to be included when the next release is cut - right now that information only exists in git.

@IvanGoncharov mentioned that this could be done by adapting a script he wrote for graphql-js: https://github.com/graphql/graphql-js/blob/master/resources/gen-changelog.js