README: add information on removal from MSFT Ubuntu feeds

As part of the migration of GCM  out of the microsoft org, we removed
packages from the Microsoft Hirsute/Bionic feeds. Adding a note to the
README informing users that these packages are no longer available and
that installing the debian package from the releases page is the preferred
mode at this time.

Author: ldennington

README.md

@@ -96,6 +96,10 @@ Download the latest [.deb package](https://github.com/microsoft/Git-Credential-M
 sudo dpkg -i <path-to-package>
 git-credential-manager-core configure
 ```
+__Note:__ Although packages were previously offered on certain
+[Microsoft Ubuntu package feeds](https://packages.microsoft.com/repos/),
+GCM no longer publishes to these repositories. Please install the
+Debian package using the above instructions instead.
 
 #### Other distributions