Document process for a project joining the org

[joshuagl]

Initially raised in slsa-framework/slsa#461

Approval should be by a majority vote if @slsa-framework/slsa-steering-committee members.

We might also consider requesting commitment to ongoing maintenance and establishing a diverse pool of maintainers.

[HyuckjunChoi]

@joshuagl I was wondering if you could let me know how it is going.