Create a writeup of how to add feature flags via flipper

[elasticspoon]

We added flipper via https://github.com/rubyforgood/casa/pull/5851

Make a wiki entry for how to use it:

• [ ] describe how to access the ui
• [ ] describe how to create a flag
• [ ] describe how to use a flag
• [ ] describe how to make a flag target a subset of users or orgs

[github-actions[bot]]

This issue has been open without changes for a long time! What's up?