Include a page about users of Rex

[ferki]

Inspired my the MogileFS users page, it would be nice to include a space on the website to collect use cases where Rex users could add themselves and a few details about their context.

Something like:

Your use case here

Basic info:

- name
- links
- using Rex since

Details:

- use case (deployment/config management/test-driven infrastructure)
- number of managed endpoints/services/groups
- number of rexfiles/modules/tasks/lines-of-code