Add requirement for cross-process cache
Add requirement for cross-process cache

Some of the trove-dashboard panels require a cross-process cache
to be used to temporarily store data until it is committed to the
server.  For example, building up the list of configuration
parameters is store in the cache until it is applied to the the
database instance.

This requirement is now added to the readme to inform users to
enable the cache.

Change-Id: I1928da600c7364d8681a8c0b45fe1c66c97032df
Closes-Bug: #1638713

openstack/trove-dashboard,openstack/trove-dashboard,openstack/trove-dashboard,openstack/trove-dashboard