docs: Update link to `create-react-app` instructions.
docs: Update link to `create-react-app` instructions.

The existing page for "Add React to a New App" (previously; now broken: https://reactjs.org/docs/add-react-to-a-new-app.html) has been moved to https://reactjs.org/docs/create-a-new-react-app.html.

This updates the documentation to accommodate that change!
apollostack/apollo-client,apollographql/apollo-client,apollostack/apollo-client,apollographql/apollo-client,apollostack/apollo-client