Update README to state a requirement for memcache

[Duk Loi]

The trove dashboard has a requirement for memcache to be configured as the cache.

For example, the configuration group parameter handling assembles the new parameters in memory before applying them via the trove api.

The readme file should reflect this requirement.

[OpenStack Infra (hudson-openstack)]

Fix proposed to branch: master
Review: https://review.openstack.org/407527

[OpenStack Infra (hudson-openstack)]

Reviewed: https://review.openstack.org/407527
Committed: https://git.openstack.org/cgit/openstack/trove-dashboard/commit/?id=230a112ee6f5d11f31d4cf89cc7ffa9e3fd0594a
Submitter: Jenkins
Branch: master

commit 230a112ee6f5d11f31d4cf89cc7ffa9e3fd0594a
Author: Duk Loi <email address hidden>
Date: Tue Dec 6 09:07:23 2016 -0500

    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 Infra (hudson-openstack)]

This issue was fixed in the openstack/trove-dashboard 8.0.0.0b2 development milestone.