Contributing docs: add hints on lumping and splitting

[ddbeck]

I've been doing a lot explaining on this question: when do I create a new feature (versus adding a key and using a compute_from and so on)?

This is my attempt to provide a general answer (which is hard!—the specifics often matter).

[captainbrosset]

I expect that we'll go back and forth on the exact wording, and that others may have slightly different ways of doing this, or additional tips. But this is a great addition and I think we should merge it.

[Elchi3]

Maybe our future selves will come up with more strict/lintable guidelines on how to create distinct web-features but until then these guidelines sound good to me.

[ddbeck]

Yes, I hope that we'll find more strict rules about these things. I think there will always be some fundamental fuzziness if we're faithfully reflecting the developer perspective, though I also hope that we'll earn some trust with developers, such that we'll be able to nudge them in helpful directions too.

Merging this now. Thank you both!