update pull_request_template.md

[rvhonorato]

You are about to submit a new Pull Request. Before continuing make sure you read the contributing guidelines and that you comply with the following criteria:

• You have sticked to Python. Please talk to us before adding other programming languages to HADDOCK3
• Your PR is about CNS
• Your code is well documented: proper docstrings and explanatory comments for those tricky parts
• You structured the code into small functions as much as possible. You can use classes if there is a (state) purpose
• Your code follows our coding style
• You wrote tests for the new code
• tox tests pass. Run tox command inside the repository folder
• -test.cfg examples execute without errors. Inside examples/ run python run_tests.py -b
• PR does not add any dependencies, unless permission granted by the HADDOCK team
• PR does not break licensing
• Your PR is about writing documentation for already existing code 🔥
• Your PR is about writing tests for already existing code

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This PR updates the pull_request_template.md (this one above) to a more informative version. We currently have a lot of checkboxes but this information is not very relevant/useful/used anywhere.