Add rudimentary pull_requests_template.md with syntax to list bids-specification-pr

Author: yarikoptic

.github/pull_request_template.md

@@ -0,0 +1,12 @@
+<!-- Fill out or delete as appropriate -->
+
+- Needs testing against a bids-specification-pr: NUMBER-or-URL
+- TODOs:
+  - [ ] ...
+
+<!-- PLEASE READ AND DELETE THE TEXT BELOW BEFORE OPENING THE PULL REQUEST -->
+
+See the [CONTRIBUTING](https://github.com/bids-standard/bids-examples/blob/master/CONTRIBUTING.md) guide. Specifically:
+
+- Please keep the title of your Pull Request (PR) short but informative - it will appear in the changelog.
+