Update docs to externalize react/addons for newer versions of React

Deprecated CommonJS react/addons entry point was removed in React 15.
https://facebook.github.io/react/blog/2016/04/07/react-v15.html
resolves #enzymejs#325

CONTRIBUTING.md

@@ -22,7 +22,7 @@ npm install && npm run react:14
 ```
 
 
-### Switching between React 0.14 and React 0.13
+### Switching between React 15, React 0.14 and React 0.13
 
 ```bash
 # switch to React 0.13
@@ -34,6 +34,11 @@ npm run react:13
 npm run react:14
 ```
 
+```bash
+# switch to React 15
+npm run react:15
+```
+
 
 ### Running Tests
 

docs/guides/browserify.md

@@ -14,6 +14,7 @@ In your browserify configuration, you simply need to make sure that the followin
 labeled as "external", which means they will be ignored:
 
 ```
+react/addons
 react/lib/ReactContext
 react/lib/ExecutionEnvironment
 ```
@@ -26,6 +27,7 @@ var browserify = require('browserify');
 var b = browserify();
 
 // make sure to mark these as external!
+b.external('react/addons');
 b.external('react/lib/ReactContext');
 b.external('react/lib/ExecutionEnvironment');
 

docs/guides/karma.md

@@ -30,6 +30,7 @@ webpack: { //kind of a copy of your webpack config
   },
   externals: {
     'cheerio': 'window',
+    'react/addons': true,
     'react/lib/ExecutionEnvironment': true,
     'react/lib/ReactContext': true
   }
@@ -49,6 +50,7 @@ browserify: {
   ],
   configure: function(bundle) {
     bundle.on('prebundle', function() {
+      bundle.external('react/addons');
       bundle.external('react/lib/ReactContext');
       bundle.external('react/lib/ExecutionEnvironment');
     });