Point bundled plugins README at the Embedded Swift sources

Author: colemancda

Sources/BluetoothExplorerPluginEngine/Plugins/README.md

@@ -3,3 +3,13 @@
 Each bundled plugin is a `<name>.wasm` module paired with a `<name>.bleplugin.json` manifest.
 These are copied verbatim into the app bundle (and the Android APK) and loaded at launch by
 `PluginManager.loadBundledPlugins()`.
+
+The modules here are build artifacts. Their source lives in `PluginSDK/Examples/`, written in
+Embedded Swift against `BLEPluginSDK`. To rebuild and reinstall one:
+
+```sh
+cd PluginSDK/Examples/BatteryLevel
+make install     # builds for wasm32, runs wasm-opt -Oz, copies here, refreshes the manifest sha256
+```
+
+See `Documentation/PluginABI.md` for the ABI and `PluginSDK/README.md` for authoring.