Add note for meeting process to CONTRIBUTING

[jonmeow]

A few things CONTRIBUTING.md could help with:

• Process, particularly "raise hands" with a link to the appropriate help page
• How to get an invite to meetings (need to ask explicitly, we won't add without explicit request -- seeing the calendar is enough to join).

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[ryoari]

has this been resolved? if not what needs to be done?

[jonmeow]

Marking closed -- I imagine the doc will keep evolving, but the process is covered elsewhere at this point.