Add dedicated storage for symlinks outside repo

.gitignore

 dotfiles/moc/pid
 dotfiles/moc/softmixer
 dotfiles/moc/playlist.m3u
+external/*

external/README.md

+This folder it intended to be ignored by git and to hold symlinks to
+paths outside this repo (possibly backed up/synced by other, more
+suitable means).
+
+dotfiles* may then safely contain relative symlinks to this sub-folder
+without need to store the--often host-specific path.