Link to "checklist" from top of admin membership console

[lilliealbert]

In-context documentation is convenient! The top of https://www.doubleunion.org/admin/members (https://github.com/doubleunion/doubleunion/blob/master/app/views/admin/members.html.haml) could link to my shiny new page of documentation for membership coordinators: https://docs.google.com/document/d/1udA9ygbX8IVop3yD8ZcsmH5_C2C2liznCLDSJvYaSg4/edit# (only visible to membership coordinators).

We'll probably want to keep editing/refining this document, so it'll probably be more useful to link it than to put the instructions at the top of the page in the app.