Add ReactJS usage instructions (#12)

- There are numerous options to how the library
  can be consumed and used within a ReactJs application
  and the examples/instructions given in the README.md
  are merely examples of how the stackdriver-error-js
  library can be instrumented within a ReactJs
  application.
- Once the library is published to npm, the instructions
  to import the module will be updated accordingly.
- Additition of a best practices section which helps
  guide users on how the library can be used in conjunction
  with Webpack and as a standalone modular utility.

Addresses #11

Author: oshalygin

README.md

@@ -119,7 +119,88 @@ Uncaught exception in angular expressions will now be reported to Stackdriver Er
 
 If you wish, you can manually delegate exceptions, e.g. `try { ... } catch(e) { $exceptionHandler(e); }` or simply `$exceptionHandler('Something broke!');`.
 
+## Setup for ReactJS
 
+Follow the general instructions denoted in _Setup for JavaScript_ to load and initialize the library.  
+
+There is nothing specific that needs to be done with React, other than making sure to initialize the library in your root entry point(typically `index.js`).
+
+## Best Practices
+
+### Only reporting in the production environment with Webpack
+
+If using webpack and the `DefinePlugin`, it is advisable to wrap the initialization logic to only occur in your production environment.  Otherwise, with local development you will receive 403s if you restricted your API key to your production environment(which is _HIGHLY_ recommended).  The code for this would look something along these lines:
+
+```javascript
+// webpack.production.js
+// The rest of your webpack configuration
+// ...
+plugins: [
+    new webpack.DefinePlugin({
+      'process.env.NODE_ENV': JSON.stringify('production')
+    }),
+  // Your other plugins
+]
+// ...
+// The rest of your webpack configuration
+```
+
+```javascript
+// index.js
+const environment = process.env.NODE_ENV; 
+
+if (environment === 'production') {
+  const errorHandler = new StackdriverErrorReporter();
+  errorHandler.start({
+    key: '<my-project-id>',
+    projectId: '<my-project-id>',
+    service: '<my-service>',              // (optional)
+    version: '<my-service-version>'       // (optional)
+  });
+}    
+```
+
+### Usage as a utility
+
+If you would like to use the error logger throughout your application, there are many options that exist.  The simplest is to pull the initialization logic into its own file and reference it as necessary throughout your application as a module.  An example would be as follows:
+
+```javascript
+// errorHandlerUtility.js
+
+const environment = process.env.NODE_ENV;
+
+let errorHandler;
+
+if (environment === 'production') {
+
+  errorHandler = new StackdriverErrorReporter(); 
+  errorHandler.start({
+    key: '<my-project-id>',
+    projectId: '<my-project-id>',
+    service: '<my-service>',              // (optional)
+    version: '<my-service-version>'       // (optional)
+  });
+
+} else {
+  errorHandler = console.error; 
+}
+
+export default errorHandler;
+
+```
+
+Consumption of the errorHandlerUtility would essentially follow the following pattern:
+
+```javascript
+// MyComponent.jsx
+import errorHandler from './errorHandlerUtility';
+
+// Some example code that throws an error
+.catch(error) {
+  errorHandler.report(error);
+}
+
+```
 
 ## FAQ