Adds documentation for Dell EqualLogic Volume Driver for Cinder

Implements: blueprint eql-vol-driver-doc
backport: havana
(cherry picked from commit acde81b97f0acf96480acf5ff4f116a00b1b2e61)

Change-Id: I70a6a169376dbd4cb2c04bf21f9b6b2c8cda95e8
Merge "Adds documentation for Dell EqualLogic Volume Driver for Cinder" into stable/havana