Add definition of member roles

[pwilkins]

The Member roles are different than those used by edX and therefore users have no context what privileges they have. Add a description of Member roles on the Members panel.

Source: Mary Ellen Wiltrout's Notes
https://docs.google.com/document/d/10iJdumDyhArCXjZVMVAtcYmpCzzqZYcsI7B9LxDg0m8/edit?pref=2&pli=1

@pdpinch