Update PULL_REQUEST_TEMPLATE.md
general improvements in template
<!--- If it fixes/closes/resolves an open issue, please link to the issue here using the template bellow (mind the link as all issues are open in the Home repository, not in this one) -->
- Fixes/Closes/Resolves nanoFramework/Home#NNNN
-## How Has This Been Tested? (if applicable)
+## How Has This Been Tested?<!-- (if applicable) -->
<!--- Please describe in detail how you tested your changes. -->
<!--- Include details of your testing environment, and the tests you ran to -->
<!--- see how your change affects other areas of the code, etc. -->
-## Screenshots (if appropriate):
+## Screenshots<!-- (if appropriate): -->
## Types of changes
<!--- What types of changes does your code introduce? Put an `x` in all the boxes that apply: -->
+- [ ] Improvement (non-breaking change that improves a feature, code or algorithm)
- [ ] Bug fix (non-breaking change which fixes an issue)
- [ ] New feature (non-breaking change which adds functionality)
- [ ] Breaking change (fix or feature that would cause existing functionality to change)
