Create settings.md

docs/docs/developer-guide/plugins/settings.md

@@ -0,0 +1,17 @@
+# Plugin Settings
+
+To add a plugin settings activity, create an activity with the following intent filter:
+
+```xml
+<activity
+    android:name=".SettingsActivity"
+    android:exported="true">
+
+    <intent-filter>
+        <action android:name="de.mm20.launcher2.action.PLUGIN_SETTINGS" />
+        <category android:name="android.intent.category.DEFAULT" />
+    </intent-filter>
+</activity>
+```
+
+Users can launch this activity by pressing the <span class="material-symbols-rounded">settings</span> on the plugin settings screen.