[WIP] docs(schedule): add release schedule & planning documentation

This change adds a docs/contributing/schedule.rst document that attempts to describe what our plans are for future Deis releases and how they are communicated. I wrote this in a matter-of-fact way, but really all the content can and should be argued over; this is just one proposal. ping @aledbf @johanneswuerbach and anyone else with an opinion, please!

Also I felt that we might want to mention the update service since it (will be) intimately tied to releases and upgrading, but on second thought maybe I should just omit that section for now...

Closes #2502.