Add a link to the docs from the README
Add a link to the docs from the README

Newcomers often struggle to find the docs for Oslo libraries. Let's
help them out a little.

Change-Id: I68cb214ad117ff1c45e83df302f37dba485147f6

dukhlov/oslo.messaging,dims/oslo.messaging,ozamiatin/oslo.messaging,isyippee/oslo.messaging,redhat-openstack/oslo.messaging,magic0704/oslo.messaging,eayunstack/oslo.messaging,magic0704/oslo.messaging,dukhlov/oslo.messaging,redhat-openstack/oslo.messaging,stevei101/oslo.messaging,dims/oslo.messaging,JioCloud/oslo.messaging,viggates/oslo.messaging,stevei101/oslo.messaging,hkumarmk/oslo.messaging,zhurongze/oslo.messaging,ozamiatin/oslo.messaging,apporc/oslo.messaging,isyippee/oslo.messaging,apporc/oslo.messaging,zhurongze/oslo.messaging,hkumarmk/oslo.messaging