Add README for tools directory
Add README for tools directory

Explain openstack-doc-tools repository

Change-Id: I19bd374560e7f89a3dc4eff3944b61a0468299d3

AlekhyaMallina-Vedams/openstack-manuals,potsmaster/openstack-manuals,Akasurde/openstack-manuals,AlekhyaMallina-Vedams/openstack-manuals,kairoaraujo/openstack-manuals,Akasurde/openstack-manuals,gongysh2004/openstack-manual,gongysh2004/openstack-manual,openstack/openstack-manuals,potsmaster/openstack-manuals,saeki-masaki/openstack-manuals,saeki-masaki/openstack-manuals,kairoaraujo/openstack-manuals,openstack/openstack-manuals,openstack/openstack-manuals,openstack/openstack-manuals,AlekhyaMallina-Vedams/openstack-manuals