Add Contributing Docs
    
        - add sign-off flag for git commit doc
        - switch to --rebase, restructure tools section for consume vs active
        - remove copr from list
        - add license to contributing.rst
        - add openshift to tools listing
        - adding ostree/rpm-ostree to contributing.rst
        - fix spelling errors in contributing.rst
        - change index to 2 depth, add contributing docs
    
    Signed-off-by: Adam Miller <admiller@redhat.com>