Add pull request template

.github/PULL_REQUEST_TEMPLATE.md

@@ -0,0 +1,35 @@
+---
+Thanks for your contribution to heroku-pipelines!
+---
+
+## Checklist
+
+The checklist enumerates the tasks you set out to do before the PR becomes ready for review
+
+- [x] Implement feature
+- [ ] Write tests
+- [ ] Update yarn.lock
+- [ ] Update [CHANGELOG.md](https://github.com/heroku/heroku-pipelines/blob/master/CHANGELOG.md)
+- [ ] Update Dev Center Docs
+- [ ] Publish a new entry to [Heroku Changelog](https://devcenter.heroku.com/changelog)
+
+## Why
+A short description of the problem being addressed or feature being added.
+
+## What are the acceptance criteria for the change?
+- [ ] [DevEx](https://github.com/heroku/devex) gives a :+1:
+- [ ] @person provides a :+1: that this PR yields the desired results
+
+## How can the change be tested
+- enumerate a *list* of steps that the reviewer may step through to fully experience the changeset being introduced
+
+## Demonstration
+- If the changeset includes user-visible results, please include before and after screenshots
+- If the changeset modifies a workflow, please a screencast/gif illustrating the interaction.
+
+## Who's Affected
+How many users will be affected by the change? Does it affect to 100% of our users, or just a subset of them?
+
+## Blockers & upstream dependencies
+- [x] Merge #1337
+- [ ] Merge API#42