Change PR template (#2224)

* Change PR template

* Update .github/PULL_REQUEST_TEMPLATE.md

* Apply suggestions from code review

Co-authored-by: Adrian Wälchli <aedu.waelchli@gmail.com>

Co-authored-by: Jirka Borovec <Borda@users.noreply.github.com>
Co-authored-by: Adrian Wälchli <aedu.waelchli@gmail.com>

.github/PULL_REQUEST_TEMPLATE.md

@@ -1,16 +1,24 @@
+
+## What does this PR do?
+
+<!--
+Please include a summary of the change and which issue is fixed. Please also include relevant motivation and context. List any dependencies that are required for this change.
+-->
+
+Fixes # (issue)
+
 # Before submitting
 
 - [ ] Was this discussed/approved via a Github issue? (no need for typos and docs improvements)
 - [ ] Did you read the [contributor guideline](https://github.com/PyTorchLightning/pytorch-lightning/blob/master/.github/CONTRIBUTING.md), Pull Request section?
-- [ ] Did you make sure to update the docs?   
-- [ ] Did you write any new necessary tests?  
+- [ ] Did you make sure your PR does only one thing, instead of bundling different changes together? Otherwise, we ask you create a separate PR for every change.
+- [ ] Did you make sure to update the documentation with your changes?
+- [ ] Did you write any new necessary tests? 
+- [ ] Did you verify new and existing tests pass locally with your changes?
 - [ ] If you made a notable change (that affects users), did you update the [CHANGELOG](https://github.com/PyTorchLightning/pytorch-lightning/blob/master/CHANGELOG.md)?
 
 <!-- For CHANGELOG separate each item in unreleased section by a blank line to reduce collisions -->
 
-## What does this PR do?
-Fixes # (issue).
-
 ## PR review    
 Anyone in the community is free to review the PR once the tests have passed.     
 If we didn't discuss your PR in Github issues there's a high chance it will not be merged.