UI heuristic checklist in the UI section of OpenStack Documentation Contributor Guide

If there's specific additions/changes to be made, please make them. We don't have bandwidth to completely overhaul this section, though, and I don't believe that the take up in Horizon (and other UI creators within OpenStack) has been enthusiastic enough to warrant a major overhaul.

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Add an OpenStack UI heuristic checklist within the OpenStack
Documentation Contributor Guide under the UI guidelines section.

Problem description
===================

The OpenStack UX project is interested in adding a section to the
OpenStack Documentation Contributor Guide that provides a
graphical user interfaces (GUI) heuristic checklist developed by the
OpenStack project teams. The content is connected to the UI text
guidelines we published for Mitaka.

Heuristic evaluations are a simple inspection method that allows
engineers and designers to review designs for common usability issues.
The benefits are that heurisitcs are low-cost and relatively easy to use
for individuals that are evaluating a design. An additonal benefit of
heurisitcs is that they help maintain consistency and transparency in how
multiple designs are reviewed.

An example of a heaurisitc inlcudes the "10 Usability Heuristics for
User Interface Design" developed by Jakob Nielsen in the 1990's that
includes principles for error recovery and system status. The OpenStack
community heurisitc includes principles that are specific to developing
for the cloud and inlcude a focus on designing to scale along with examples
such as the number of images within a typical production deployment.

The working draft of these guidlines is in progress and can be viewed
here: https://etherpad.openstack.org/p/mitaka-openstackux-heuristic

The value of this effort is to help provide an improved
usability for operators, admins and end users by creating
tools that help promote consistency in OpenStack GUI interfaces.