Add package.json scripts ro run android and ios in template (#25618)

Summary:
Currently users have to use the global `react-native` command or run `yarn react-native run-x` which isn't very nice.

This PR adds `android` and `ios` scripts to `package.json` so users can run `yarn android` or `yarn ios` directly.
Pull Request resolved: https://github.com/facebook/react-native/pull/25618

Differential Revision: D16223229

Pulled By: cpojer

fbshipit-source-id: 69b082760ff2ee31ab0406251f9a50339fd227bf

template/package.json

@@ -3,6 +3,8 @@
   "version": "0.0.1",
   "private": true,
   "scripts": {
+    "android": "react-native run-android",
+    "ios": "react-native run-ios",
     "start": "react-native start",
     "test": "jest"
   },