Merge pull request #3046 from matrix-org/bwindels/settings-config-doc

document settingDefaults

docs/settings.md

@@ -51,6 +51,17 @@ Settings are the different options a user may set or experience in the applicati
 }
 ```
 
+Settings that support the config level can be set in the config file under the `settingDefaults` key (note that some settings, like the "theme" setting, are special cased in the config file):
+```json
+{
+  ...
+  "settingDefaults": {
+    "settingName": true
+  },
+  ...
+}
+```
+
 ### Getting values for a setting
 
 After importing `SettingsStore`, simply make a call to `SettingsStore.getValue`. The `roomId` parameter should always