Adds repo migration note
As per CGDS policy (https://www.wrike.com/open.htm?id=887986488), a note is added to readme stating that this repo is now hosted at github. After merging this MR to the master, the repo will be archived as per the policy.
As per CGDS policy (https://www.wrike.com/open.htm?id=887986488), a note is added to readme stating that this repo is now hosted at github. After merging this MR to the master, the repo will be archived as per the policy.