Add docs on $?DISTRIBUTION
Add docs on $?DISTRIBUTION

Requires https://github.com/rakudo/rakudo/pull/2574

MasterDuke17/doc,MasterDuke17/doc,perl6/doc,perl6/doc,tbrowder/doc,MasterDuke17/doc,perl6/doc,tbrowder/doc,tbrowder/doc,tbrowder/doc,perl6/doc,MasterDuke17/doc,perl6/doc