Contributing Guide Bevy Organization follow-up

[TrialDragon]

1. Shift focus on release candidate process to include an emphasis on preparing the release. From Add chapter on the Bevy Organization to the Contributing Guide #1596 (comment) and Add chapter on the Bevy Organization to the Contributing Guide #1596 (comment)

2. Note that maintainers are also board members. From Add chapter on the Bevy Organization to the Contributing Guide #1596 (comment)

[alice-i-cecile]

I've updated the description to capture what the follow-up items actually are :)