Update .gitignore

Author: erikw

.gitignore

@@ -2,7 +2,7 @@ segments/np_mpd
 segments/xkb_layout
 *.swp
 
-# ignore all themes, only default.sh
+# ignore all themes, except for the built-in ones. User themes should be put in `~/.config/tmux-powerline/themes/
 themes/*
 !themes/default.sh
 !themes/bubble_theme.sh