added explaination for bspc rules

.config/bspwm/bspwmrc

@@ -16,6 +16,9 @@ bspc config urgent_border_color "#B58900"
 bspc config presel_border_color "#26E2FF"
 
 # some rules
+# to get the name, search WM_CLASS(STRING) in xprop
+# and pick the second entry
+# e.g. WM_CLASS(STRING) = "zathura", "Zathura"
 bspc config external_rules_command "$XDG_CONFIG_HOME/bspwm/external-rules"
 bspc rule -a Gimp desktop='^8' state=floating follow=on
 bspc rule -a mplayer2 state=floating