Expand docs describing project governance and development cycle

[gerth2]

Timeline and development came up more than once in the last 12 months. Adding some basic info to the "contributing" page to explain how the team is organized and how activities are prioritized.

Ensure there's verbiage about being "descriptive, not prescriptive". The goal is just to get someone who's on the "outside" a flavor of how things run on the "inside" - not to dictate process or procedrues.