Add GitHub pull request template

Author: Cito

.github/PULL_REQUEST_TEMPLATE.md

@@ -0,0 +1,15 @@
+Thank you for contributing to GraphQL-core!
+
+If your pull-request is non-trivial, adds a feature or contains a non-breaking change,
+then please, first open an issue to discuss the proposed changes and add a link to it.
+
+GraphQL-core tries very hard to stay within the scope of being just a Python port
+of [GraphQL.js](https://github.com/graphql/graphql-js).
+
+Any additional feature  or incompatible change will be only accepted in rare cases
+and require compelling reasons, because they aggravate maintenance and synchronization
+with the developments in the upstream project. So please discuss such changes upfront
+in an issue before sending a PR. Maybe there are other ways to solve the problem.
+
+If possible, also add unit tests, or provide runnable example code as part of the
+accompanying issue, from which unit tests can be derived.