Link to readthedocs for the installation guide
Link to readthedocs for the installation guide

Since we use Sphinx for docs, like other projects, the github link is
no longer working, and even if it was working, it wouldn't render the
installation guide correctly. Linking to readthedocs instead.

Change-Id: I6090af6cf7363fea0956cd731f717a6dcd1b5811

rdo-management/tuskar-ui,rdo-management/tuskar-ui,rdo-management/tuskar-ui,rdo-management/tuskar-ui