[Gnu-morello] [binutils-gdb] Add README-morello file to binutils repo

Hi all,

This patch adds a README-morello file that describes the users/ARM/morello-binutils-gdb-master branch, its maintainers and points to this list.

Pushed to the sourceware repo on the users/ARM/morello-binutils-gdb-master branch.

Thanks, Kyrill