[type casting & coercion] add note about bitshift and large integers. fixes airbnb#109

Author: hshoff

README.md

@@ -956,6 +956,7 @@
     ```
 
   - If for whatever reason you are doing something wild and `parseInt` is your bottleneck and need to use Bitshift for [performance reasons](http://jsperf.com/coercion-vs-casting/3), leave a comment explaining why and what you're doing.
+  - **Note** Be careful when using bitshift operations. Numbers are represented as [64-bit values](http://es5.github.io/#x4.3.19), but Bitshift operations always return a 32-bit integer [source](http://es5.github.io/#x11.7). Bitshift can lead to unexpected behavior for integer values larger than 32 bits. [Discussion](https://github.com/airbnb/javascript/issues/109)
 
     ```javascript
     // good