update CONTRIBUTING.md, mention plugins

[bajtos]

Quoting from #1454:

Users get upset when you don't merge their PRs. I think contributing to this is that users do not have a clear, documented way to develop a plugin, so they feel like the only way that their feature will ever see the light of day is if it gets merged.

This PR updates CONTRIBUTING.md and adds a note that people should try to write new features as plugins.

[boneskull]

Thanks for this, but part of the problem there isn't much of a plugin API! See #1457

[bajtos]

I see, it makes sense. Feel free to close this PR as rejected in that case.