Add PR template with links to contrib docs

docs/pull_request_template.md

@@ -0,0 +1,7 @@
+<!--
+
+[Our guide to getting a PR merged](https://repo2docker.readthedocs.io/en/latest/contributing/contributing.html#guidelines-to-getting-a-pull-request-merged).
+
+About to propose a big change? Read [code contributions](https://repo2docker.readthedocs.io/en/latest/contributing/contributing.html#process-for-making-a-code-contribution)
+to maximise the chances of it getting merged quickly.
+-->