Add deployment checklist to `CONTRIBUTING.md`

[vdusek]

The problem it should solve

• Our Check & Release / Publish to PyPI (push) often fails duo to a forgotten update of CHANGELOG.md or some other step.
• Our releases sometimes contain inconsistencies (tag name, release name).

Solution

• Add some sort of checklists (one for beta release, one for release) to the https://github.com/apify/apify-sdk-python/blob/master/CONTRIBUTING.md.
• Add the same to our other Python repositories (https://github.com/apify/apify-client-python, https://github.com/apify/apify-shared-python) - it could be just a link to this repository.