doc: sketch out pull request template (ooni#437)

My main concern here is to remind contributors (_including myself_) about the documents they should have read and that every change may entail also changes in ooni/spec. This work counts as a cleanup before closing ooni/probe#1729.

PULL_REQUEST_TEMPLATE.md

@@ -0,0 +1,9 @@
+## Checklist
+
+- [ ] I have read the [contribution guidelines](https://github.com/ooni/probe-cli/blob/master/CONTRIBUTING.md)
+- [ ] reference issue for this pull request: REFERENCE_ISSUE_URL
+- [ ] related ooni/spec pull request: RELATED_OONI_SPEC_PULL_REQUEST_URL
+
+## Description
+
+Insert here a more detailed description (if applicable).