Add a git repo for security documentation

This adds a new git repository for documentation created by the OpenStack
Security Group (OSSG). This will be a common place for holding the OSSNs,
the Security Guide, and potentially future documentation output from OSSG.

This work lives under the Documentation program. See the launchpad bug
for other details and discussion.

Change-Id: Ie72134e716ec800d8bc601167a8161a605a9cd5f
Closes-bug: 1319895
Merge "Add a git repo for security documentation"