Setup for openstack-doc-tools

Add files to allow us gating via:
tox -e checkbuild
tox -e checkniceness
etc

Add .gitignore to ignore files used by gating.

Add initial README file to document this

Change-Id: I821e1848bb364f7514c54e87f5d29d4254b9e025
Merge "Setup for openstack-doc-tools"