Add a README before publishing this package

Summary:
Before we can publish `0.0.1` of `react-native-gradle-plugin`, we
need to write a README file for it.

Changelog:
[Internal] [Changed] - Add a README before publishing the react-native-gradle-plugin

Reviewed By: ShikaSD

Differential Revision: D31206204

fbshipit-source-id: 0168298d04d4619dde931eb75d9555982f50fffe

packages/react-native-gradle-plugin/README.md

@@ -0,0 +1,16 @@
+# react-native-gradle-plugin
+
+[![Version][version-badge]][package]
+
+A Gradle Plugin used to support development of React Native applications for Android.
+
+## Installation
+
+```
+yarn add react-native-gradle-plugin
+```
+
+*Note: We're using `yarn` to install deps. Feel free to change commands to use `npm` 3+ and `npx` if you like*
+
+[version-badge]: https://img.shields.io/npm/v/react-native-gradle-plugin?style=flat-square
+[package]: https://www.npmjs.com/package/react-native-gradle-plugin

packages/react-native-gradle-plugin/package.json

@@ -20,7 +20,8 @@
     "gradle",
     "gradlew",
     "gradlew.bat",
-    "src"
+    "src",
+    "README.md"
   ],
   "dependencies": {
     "react-native-codegen": "*"