Create release schedule for stable

Create an schedule for stable releases and document steps on how stable releases should be considered and accepted (release candidates, minor test period required, etc).

Out of the top of my head I can think of the following requirements for a release to be considered as stable:

• At least a week of being released
• No bugs reported caused by that release

Also to consider:

• Release candidates for stable releases, major versions

Input on this is greatly appreciated.

See https://github.com/zsh-users/antigen/issues/524#issuecomment-302773206