Formalize process for major releases

[reedsa]

Update the contributing docs with information about major releases. This is not intended to be an exhaustive list but is something we can work on as we work toward v7.

Interesting topics to explore:

• how are plans are created for the next major release (using the roadmap, issues, community input, design docs)?
• how do we use beta/release branches?
• when should we create a new major version branch?
• when do we conclude betas and do the proper release?
• when and how do we sunset a major version?
• how should each milestone be communicated?