[OUTREACHY] Added a submitting a proposal doc to maintenance

[Gábor Boskovits]

Hello guix,
I added a submitting a proposal file to maintenance, that
describes what the community specific fields should look like,
and also some recommendations on the rest. The project specific fields
and the mentor specific fields can't be filled in form these, but I
still
believe this can help to shorten this process.

Best regards,
g_bor
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