Clarify Stage 0 proposals requirements

[Ginden]

Process documents seems to be invalid in regard of champion required. See tc39/proposals#71 for details.

[ljharb]

Repeating my comment from tc39/proposals#71 (comment):

The committee has been very clear that non-members can not bring any proposal without a member sponsoring it. Note, however, that's not necessarily the same as a champion.

In other words, stage 0 does not require a champion, but it still requires a TC39 member's cooperation.

[littledan]

Please let me know if you are looking for a champion for a proposal. There are many in the committee who are interested in helping out community members here.

[Ginden]

@littledan I will be looking for champion after completing spec text for new Set methods.

Regardless, process document is misleading here.