Add notes to PR template

[magicznyleszek]

Checklist

1. If you've added code that should be tested, add tests
2. If you've changed APIs, update (or create!) the documentation
3. Ensure the tests pass
4. Make sure that your code lints and that you've followed our coding style
5. Write a description of your work suitable for publishing on our forum
6. Mention any related issues in this repository (as #ISSUE) and in other repositories (as kobotoolbox/other#ISSUE)
7. Open an issue in the docs if there are UI/UX changes

Description

Following up on the Front end technical call, I'm adding the "Notes" section to PR template with a short description.

[jnm]

I've added more details and changed placeholders from HTML comments to regular text. I've found that people would simply leave those comments in place instead of writing what they were supposed to write.