Add steel door/trapdoor sounds.

I've found a favorable steel door sound from a parking garage
door that isn't abrupt or scary, just sounds like a nice solid
metal door. The sample had both opening and closing sounds, and
so they match nicely. Amplified and mixed several samples together
to reduce ambient noise, and get the right level compared to
wood doors. Attribution was added as well. CC-BY-3.0 sounds.

mods/doors/README.txt

@@ -75,4 +75,7 @@ fencegate_open.ogg:
 fencegate_close.ogg:
   http://www.freesound.org/people/BarkersPinhead/sounds/274807/ - CC-BY-3.0
   http://www.freesound.org/people/rivernile7/sounds/249573/ - CC-BY-3.0
-
+Steel door sounds (open & close (CC-BY-3.0) by HazMatt
+  - http://www.freesound.org/people/HazMattt/sounds/187283/
+  doors_steel_door_open.ogg
+  doors_steel_door_close.ogg