(Emacs)~Stop tracking custom.el

I don't like custom.el.  I don't like what it does.  Anything in it
that's actually useful should be made persistent in my configuration
anyway.  So why should I have it tracked in git?  Should be set per
machine anyway.

.gitignore

@@ -21,4 +21,5 @@ TAGS
 /Mail/.local/share/mail/
 /Pictures/
 /SystemD/.config/systemd/user/default.target.wants/
-/SystemD/.config/systemd/user/timers.target.wants/
+/SystemD/.config/systemd/user/timers.target.wants/
+Emacs/.config/emacs/.config/custom.el