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...~~

I'll leave this PR open for a while (days) until we've exhausted debate and tweaked it. I'm going to remove the mention of the update service.

Closes #2502.