Suggest that other stage advancement requirements can be articulated, specific to a proposal

[littledan]

We discussed #15 in TC39, and several committee members (including @wycats @ljharb @zbraniecki and others) had this great suggestion: When moving from one stage to the next, or any time later, the committee and champions may articulate further things that need to be done to move to the following stage. We might want to write this down in the process document, or in a separate guide for how to champion proposals/be a committee member. We've been doing this sometimes, often in earlier stages; it may be useful for advancing from Stage 3 to 4 as well, articulating web compatibility, implementer experience and buy-in requirements specific to the proposal.

[ljharb]

I'd be happy to write up a first-draft PR in the next few months.

[littledan]

Great! I'd suggest to keep it open ended; a list of possible considerations could be helpful, but to fully document the kinds of goals TC39 goes for might be better done elsewhere.