Updated babel-preset-react-app README.md (facebook#3463)

Added some more documentation to install the babel-preset-react-app making it more clear on how to get started with this preset outside of create-react-app.

Author: boyney123

packages/babel-preset-react-app/README.md

@@ -16,6 +16,12 @@ If you want to use this Babel preset in a project not built with Create React Ap
 
 First, [install Babel](https://babeljs.io/docs/setup/).
 
+Then install babel-preset-react-app.
+
+```sh
+npm install babel-preset-react-app --save-dev
+```
+
 Then create a file named `.babelrc` with following contents in the root folder of your project:
 
   ```js