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.
Gitlab has been upgraded from 17.11.7 to 18.2.8. See the following release notes: - Gitlab 18.0 - Gitlab 18.1 - Gitlab 18.2
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.