Add Community Contributions section to README

[CooperBigFoot]

Summary

• Adds a "Community Contributions" section at the end of README.md
• Links to watershed-retrieve, a community package providing pre-delineated MERIT-Hydro watershed boundaries for ~60,000 gauging stations
• References [Feature] Add MERIT-Hydro watershed boundaries for ~58k gauges #87 for the original feature proposal

Details

Since index.rst already includes README.md via myst-parser, this change automatically appears in the Sphinx-rendered documentation on ReadTheDocs as well. No changes to docs/ are needed.

This is a docs-only change — no code modifications.

Test plan

• CI docs build (make html) passes
• Section renders correctly on GitHub (preview in PR "Files changed" tab)
• Section renders correctly on ReadTheDocs after merge

[kratzert]

Looks good to me. We might want to shorten the section a bit if at any point we would have more community packages to link, but for the moment I would keep it as is.

Thanks again, I'll merge this into main