Add primer on feature flags to contrib docs

docs/contribute/feature-flags.md

@@ -0,0 +1,92 @@
+---
+navTitle: Working with Feature Flags
+---
+
+When adding features to the platform it is sometimes a requirement to be able to
+restrict the feature to licensed platforms, and furthermore to certain types of team
+on the platform.
+
+Most typically this will be a feature that should only be available to `Team`
+or `Enterprise` tiers on FlowFuse Cloud.
+
+This is a quick guide for how to add a feature flag - both at the platform-wide
+level and against individual `TeamTypes`.
+
+Feature flag names should use `camelCase`.
+
+### Add a platform-wide feature flag
+
+All features that are licensable should set a platform-wide feature flag to indicate
+the feature is available.
+
+```
+app.config.features.register('featureFlagName', true)
+```
+
+[Here](https://github.com/FlowFuse/flowfuse/blob/0335c9056019ff9987d97f3ad3f18675de1c2422/forge/ee/lib/ha/index.js#L6) is
+an example of how the HA feature sets its platform-wide flag.
+
+
+### Add a team type feature flag
+
+The feature flag is set by an admin user via [`TeamTypeEditDialog.vue`](https://github.com/FlowFuse/flowfuse/blob/0335c9056019ff9987d97f3ad3f18675de1c2422/frontend/src/pages/admin/TeamTypes/dialogs/TeamTypeEditDialog.vue).
+
+
+1. Add an entry to the existing list of feature flags [here](https://github.com/FlowFuse/flowfuse/blob/0335c9056019ff9987d97f3ad3f18675de1c2422/frontend/src/pages/admin/TeamTypes/dialogs/TeamTypeEditDialog.vue#L73-L85).
+2. Add a check to ensure the right default value is applied [here](https://github.com/FlowFuse/flowfuse/blob/0335c9056019ff9987d97f3ad3f18675de1c2422/frontend/src/pages/admin/TeamTypes/dialogs/TeamTypeEditDialog.vue#L172-L174). Any new feature should default to `false` so it can then be selectively enabled.
+
+### Using the feature flags - runtime side
+
+Platform-wide feature flags can be checked using:
+
+```
+const isFeatureEnabledOnPlatform = app.config.features.enabled('featureFlagName')
+
+```
+
+`TeamType` feature flags can be checked using the `getFeatureProperty` function
+of the `TeamType` model:
+
+
+```
+// myTeamType is an instance of `TeamType`
+const isFeatureEnabledForTeamType = myTeamType.getFeatureProperty('featureFlagName', false)
+```
+
+The first arg is the name of the feature flag, the second arg is the default value
+if the feature flag is otherwise unset. As mentioned above, any new feature should
+default to `false`.
+
+[Here](https://github.com/FlowFuse/flowfuse/blob/0335c9056019ff9987d97f3ad3f18675de1c2422/forge/ee/routes/sharedLibrary/index.js#L22) is an example of this in action.
+
+### Using the feature flag - frontend
+
+In the frontend, platform-wide feature flags can be checked against
+the `features` property of the `account` store.
+
+TeamType feature flags can be checked against `team.type.properties.features.featureFlagName`.
+
+[Here](https://github.com/FlowFuse/flowfuse/blob/0335c9056019ff9987d97f3ad3f18675de1c2422/frontend/src/pages/application/DeviceGroups.vue#L135-L146) is an example of how we combine these two things:
+
+```
+    computed: {
+        ...mapState('account', ['features']),
+        featureEnabledForTeam () {
+            return !!this.team.type.properties.features?.deviceGroups
+        },
+        featureEnabledForPlatform () {
+            return this.features.deviceGroups
+        },
+        featureEnabled () {
+            return this.featureEnabledForTeam && this.featureEnabledForPlatform
+        }
+    },
+```
+
+This allows the UI to distinguish between a feature being unavailable because the platform
+is not licensed for it, and a feature being unavailable for the current team type.
+
+This allows different messages to be display with the most appropriate call to action.
+
+The `EmptyState` component has support for this - see [here](https://github.com/FlowFuse/flowfuse/blob/0335c9056019ff9987d97f3ad3f18675de1c2422/frontend/src/pages/application/DeviceGroups.vue#L29)
+for how that is applied.