Included the contribution checklist section in the PR template

[bgorski]

Description (*)

With the introduction of the README.md template, there is a requirement of describing module installation, structure and extension points in the readme.
We need to pay special attention to README.md always being up to date, because an outdated readme can do more harm than good. This is why all code contributors need to be aware that they are supposed to modify README.md as well when they create modifications that affect any of readme predefined sections.
Also, because this repository didn't have the contribution checklist section at all, I added it.

Fixed Issues (if relevant)

1. Fixes magento/adobe-stock-integration#<issue_number>: Issue title
2. ...

Manual testing scenarios (*)

1. Attempt to create a Pull Request to this or any other github repository
2. When on the PR edit page, copy-paste the modified template content into your PR description area
3. Click Preview
4. Along the Contribution checklist checkboxes you should see a new checkbox with a description saying that README.md needs to be modified if necessary
5. Check if it looks good
6. Don't create that PR as you don't need to, unless you want to check that the markdown looks the same on an actual PR as in the Preview window