Tweak the PR check-list and add more information about how to contribute

I've updated the PR check-list to make it more readable. That should also help the review process as spotting BC breaks for instance or any other information should be easier and faster.

I've also added a small tip about how to create a todo-list on Github.

You see the new check-list in action here:

https://github.com/symfony/symfony/pull/6738

ping @vicb