contact.rst: correct email addr. for dev list

The email address for both the general discussion and dev lists were
zerovm@googlegroups.com. The correct address for the dev list is
zerovm-devel@googlegroups.com.
contact.rst: correct grammar

The google group instructions ask people to "signup". "Signup" is a
noun, not a verb. The correct way to write this is "sign up".
Merge pull request #23 from larsbutler/fix-contact

Fixes to `contact` page