tox -e releasenotes fails with:
Warning, treated as error:
WARNING: html_static_path entry
u'/home/aj/vcs/OpenStack/openstack/cursive/releasenotes/source/_static'
does not exist
Add missing directory - like done on similar projects -,
using a placeholder to make git happy.
Also, add build directory to .gitignore.
Remove .placeholder from .gitignore, those files are important!
Change-Id: I20d9881975eb491f6a977ae2de9406e64a659ca9