Update somewhat stale and occasionally over-colloquial website guidance on
new committer proposals to:

• Provide more details on the contents of a suitable proposal, including a request for evidence of constructive engagement with the mentor (e.g., through successfully completed patch cycles).
• Be more overt about identifying community participation (but also more open about what it means -- mention BSD-conference talks, for example).
• Suggest contacing core@ informally first if there are worries about whether there is sufficient contribution or concern about a proposal failing.
• Expand thinking on 'vendor commit bits' -- both the responsibilities of the mentor, and also the hope that such contributors will broaden their interests over time.

Approved by: core