update PR template

Author: wilhelmberg

PULL_REQUEST_TEMPLATE.md

@@ -1,12 +1,20 @@
+**Related issue**
 
+Example: Closes #832. Relates to #832.
 
+**Description of changes**
 
+Your text here!
 
+**QA checklists**
 
+- [ ] Add relevant code comments. Every API class and method should have `<summary>` description as well as description of parameters.
+- [ ] **Add tests for new/changed/updated classes and methods!!!**
+- [ ] Check out conventions in [CONTRIBUTING.md](https://github.com/mapbox/mapbox-unity-sdk/blob/develop/CONTRIBUTING.md).
+- [ ] Check out conventions in [CODING-STYLE.md](https://github.com/mapbox/mapbox-unity-sdk/blob/develop/CODING-STYLE.md)
+- [ ] Update the [changelog](https://github.com/mapbox/mapbox-unity-sdk/blob/develop/documentation/docs/05-changelog.md)
+- [ ] Update documentation.
 
+**Reviewers**
 
-
-___
-_As part of the process of submitting this PR, please:_
-- [ ] Document the PR changes
-- [ ] Update the changelog
+Tag your reviewer(s). Choose wisely.