message: Update the React readme
  I'm going to fetch this readme for use in airbrake-docs.
  I've decreased the size of the header to better match what we use on the
  docs site and added a bit more copy explaining the setup.
diff: -# React error handler
  +### React error handler
  -You need to define `ErrorBoundary` component:
  +To report errors from a React app, you'll need to set up and use an
  +[`ErrorBoundary` component](https://reactjs.org/blog/2017/07/26/error-handling-in-react-16.html)
  +and initialize an `AirbrakeClient` with your `projectId` and `projectKey`.
  ```js
  import AirbrakeClient from 'airbrake-js';
  