dillon1000/react

document checkbox using click handlers for change events

I hit an issue related to this earlier today. Feels like a short paragraph warning of potential issues would be of benefit.  Discussed with @zpao on IRC, agreed to make PR.

For more info:
* https://github.com/facebook/react/issues/3005#issuecomment-72513965
* https://github.com/facebook/react/blob/c7e4f55eb061b578497ed316d3e9e0dfa7026f46/src/browser/eventPlugins/ChangeEventPlugin.js#L287