android build [nfc]: Comment with "ram-bundle" example.

Part of the RN v0.59 -> v0.60 changes to the template app [1]. This
commit may freely be positioned before the upgrade; a guide to this
setting has been available in RN v0.59 [2].

Corresponds to facebook/react-native@4478e5052, released in RN
v0.60.0.

[1]: https://react-native-community.github.io/upgrade-helper/?from=0.59.10&to=0.60.6

[2]: https://reactnative.dev/docs/0.59/performance#enable-the-ram-format

android/app/build.gradle

@@ -22,6 +22,9 @@ import com.android.build.OutputFile
  *   // the entry file for bundle generation
  *   entryFile: "index.android.js",
  *
+ *   // https://facebook.github.io/react-native/docs/performance#enable-the-ram-format
+ *   bundleCommand: "ram-bundle",
+ *
  *   // whether to bundle JS and assets in debug mode
  *   bundleInDebug: false,
  *