Ask for commit after changelog and author edit in contributing guidelines

[charlesaracil-ulti]

In the contribution guidelines (https://github.com/charlesaracil-ulti/pytest/blob/master/CONTRIBUTING.rst#long-version), section 8 asks to commit, and then offers add a changelog and edit the authors file. So following those guideline verbatim will have those last two items excluded from the push.

I might be better to ask for commit after changelog and authors file edit.

• a detailed description of the bug or problem you are having
• output of pip list from the virtual environment you are using
• pytest and operating system versions
• minimal example if possible