Gateway developer documentation on how to "Login with XSEDE"

The XSEDE Cyberinfrastructure Integration (XCI) team has completed writing and testing the document "User Authentication Service for XSEDE Science Gateways." This is an introduction to the user authentication service that XSEDE offers for science gateway developers and operators. This service provides a user "login" function so that gateway developers don't need to write their own login code or maintain user password databases.