contributing.md: suggest multi-repo features target main

[scpeters]

🎉 New feature

Adds recommendation about interdependent multi-repository features

Summary

I believe we discussed somewhat recently a recommendation to target interdependent multi-repository features to main instead of release branches since this simplifies CI / automated testing and pull request review, so I've added that recommendation to our contribution guide.

Test it

Preview and proofread the changes

Checklist

• Signed all commits for DCO
• Added tests
• Added example and/or tutorial
• Updated documentation (as needed)
• Updated migration guide (as needed)
• Consider updating Python bindings (if the library has them)
• codecheck passed (See contributing)
• All tests passed (See test coverage)
• While waiting for a review on your PR, please help review another open pull request to support the maintainers

Note to maintainers: Remember to use Squash-Merge and edit the commit message to match the pull request summary while retaining Signed-off-by messages.