Add instructions on managing monitor feature flags via spreadsheet/table

Monitor has had some issue come up around managing feature flags: https://gitlab.com/gl-retrospectives/monitor/issues/21

We've haven't had a formalized process for managing flags to date, though some folks have been using https://docs.google.com/spreadsheets/d/1oNY3NibGDXhnA-GJ6TD4Q6fuRs1p-v3AZNljupkg8pI/edit#gid=0 to track the status of flags.

This proposes making the process of addition feature flags to a single table and adds a step of communication via slack.

An alternate proposal to manage flags via issues can be found here: !32532 (closed)