Updated CONTRIBUTING.md

[Ali-RS]

Added "Customs around integration, branching, tagging, and releases" section to contribution guide.

[stephengold]

I'm in favor of integrating this addition!

Though perhaps we should clarify that the admonition to squash PRs is directed at integrators, not submitters.

[Ali-RS]

You mean something like this:

Integrators should note, unless the history of the pull request is important, it should be integrated to a single commit using “squash and merge”. If the history is important, favor “rebase and merge”.
Don’t create a merge commit unless GitHub cannot rebase the PR.

[stephengold]

Yeah, like that. Thanks.

[Ali-RS]

Done.