Updated Duo with supported features.
Add Zendesk to the list.

People have been asking Zendesk for 2-factor authentication since at
least 2011. So far the company has neglected to implement support for it
going as far as telling people to 'use Google SSO' or 'do it yourself
through SAML'.
Merge pull request #413 from mxxcon/Duo

Updated Duo with supported features. Closes #412
Merge pull request #414 from daenney/zendesk

Add Zendesk to the list.
Add doc link to Developer/RamNode
Merge pull request #415 from mxxcon/RamNode

Add doc link to Developer/RamNode