Update change management documentation for new release and development policies

[rjyounes]

Release schedule

• Major versions: Released on an ad hoc basis when we have a major change to release, but no more than once every 6 months. (We actually said no more than twice a year, but I think guaranteeing a 6-month spacing is preferable.) The goal is to release a new major version at least once a year.
• Minor version: Quarterly releases (still probably aspirational).

Development

• Continue biweekly full group meetings for triage, oversight, and release planning.
• Form ad hoc working groups to deal with large changes – mostly major, but sometimes a large number of smaller changes that are impractical to work out in a group, such as ontology-wide annotation cleanup.
• Each group should govern their own working schedule/process, though selecting a facilitator is recommended for efficient functioning.
• Once the group comes to agreement, they will submit a PR and present their solution to the entire group in one meeting (or in the reverse order, i.e., presentation preceding PR).
• The large group can propose revisions, which the small group will consider but may or may not adopt.
• The PR gets incorporated into the release schedule.