Update PULL_REQUEST_TEMPLATE.md

I've copied and modified the k6 PR template for our purposes. Let me know if it makes sense. Thanks!

.github/PULL_REQUEST_TEMPLATE.md

@@ -1,10 +1,32 @@
-### Description of changes
+## What?
 
-Please describe your pull request.
+<!-- A short (or detailed) description of what this PR does. -->
 
-### Checklist
+## Why?
 
-- [ ] Written tests for the changes
-- [ ] Update k6 documentation (if relevant) -- PR link: ?
-- [ ] Update [k6 browser get started](https://k6.io/blog/get-started-with-k6-browser/) blog (if relevant) -- PR link: ?
-- [ ] Generate and update [TypeScript definitions](https://github.com/DefinitelyTyped/DefinitelyTyped/tree/master/types/k6/experimental/browser) (if relevant)
+<!-- A short (or detailed) explanation of why these changes are made and needed. -->
+
+## Checklist
+
+<!-- 
+If you haven't read the contributing guidelines https://github.com/grafana/k6/blob/master/CONTRIBUTING.md 
+and code of conduct https://github.com/grafana/k6/blob/master/CODE_OF_CONDUCT.md yet, please do so
+-->
+
+- [ ] I have performed a self-review of my code
+- [ ] I have added tests for my changes
+- [ ] I have commented on my code, particularly in hard-to-understand areas
+
+<!--
+If relevant:
+- [ ] I have updated the k6 release notes: PR link
+- [ ] I have updated the k6 documentation: PR link
+- [ ] I have updated [k6 browser get started](https://k6.io/blog/get-started-with-k6-browser/) blog: PR link
+- [ ] Generated and updated the [TypeScript definitions](https://github.com/DefinitelyTyped/DefinitelyTyped/tree/master/types/k6/experimental/browser)
+-->
+
+## Related PR(s)/Issue(s)
+
+<!-- Does it close an issue? -->
+
+<!-- Closes #ISSUE-ID -->