Add reasoning for using double quotes in JSX

Author: cooperka

react/README.md

@@ -117,6 +117,9 @@
 
 ## Quotes
   - Always use double quotes (`"`) for JSX attributes, but single quotes for all other JS.
+
+  > Why? JSX attributes [can't contain escaped quotes](http://eslint.org/docs/rules/jsx-quotes), so double quotes make conjunctions like `"don't"` easier to type.
+
     ```javascript
     // bad
     <Foo bar='bar' />