Added Ruby gems script

Necessary to provide a single source of truth for installing Ruby
dependencies. This used to be managed by the Ruby Setup project but
that project is being deprecated in favor of this project.

bin/run

@@ -39,6 +39,7 @@ while true; do
     printf "     m:  Install Mac App Store software.\n"
     printf "     a:  Install application software.\n"
     printf "     x:  Install application software extensions.\n"
+    printf "    rg:  Install Ruby gems.\n"
     printf "    rc:  Install Rust crates.\n"
     printf "     d:  Apply default settings.\n"
     printf "     s:  Setup installed software.\n"