Document how to use feature flags with flipper

docs/flipper-feature-flags.md

@@ -0,0 +1,31 @@
+Feature Flags
+=============
+
+Feature flags allow specific features to be turned on and off at runtime for a
+percentage of users or the enitire site. Feature flags enable:
+
+- Testing out new features on a limited portion of our users
+- Quick rollback when we discover a problem in production
+
+Process
+-------
+
+1. Surround the new feature's code with a conditional:
+
+    ```ruby
+    if $flipper[:a_name_for_your_feature].enabled?(current_user)
+      # do the new thing
+    else
+      # do the old thing, or maybe nothing
+    end
+    ```
+
+2. In development, enable the feature in the UI (http://localhost:4567/flipper/)
+3. Get your feature deployed, ensuring you mention the feature flag in the Pull
+   Request. Ask a flipper admin (@kytrinyx) to enable your feature.
+4. Ensure everything is working in production.
+5. **Important**: Create a new pull request that removes all your feature flipper's
+   conditionals, removing any old code the new feature replaced and all references to
+   your feature flag. Leaving enabled feature flags in the code increases complexity
+   and is unacceptable. The feature flag admin will delete the feature flag in the UI
+   after the PR is merged and deployed successfully.