Contact Form: add help message to explain to send to multiple emails

It's currently possible to send out notification emails to multiple email addresses by separating email addresses with commas in the form builder. But we don't mention this anywhere in the docs. This commit adds an extra help field to the right of the form builder:

![screen shot 2014-03-10 at 10 14 29 am](https://f.cloud.github.com/assets/426388/2372059/d4599a16-a834-11e3-8668-6165421c0774.png)

Reported here:
* https://twitter.com/jeffr0/status/441332282058887169