Add sound licenses for mech mod.

mods/mech/sounds/license.md

@@ -0,0 +1,15 @@
+- `button_trigger.ogg`
+- `button_untrigger.ogg`
+- `piston_trigger.ogg`
+- `piston_untrigger.ogg`
+
+ Artist: Alecia Shepherd
+ See: http://www.aleciashepherd.com
+ Origin: https://www.minecraftforum.net/forums/mapping-and-modding-java-edition/resource-packs/1245112-snowsong-the-epic-sound-pack-sound-resource-pack
+ License: CC-BY
+
+- `nexus_place.ogg`
+ Artist: Iwan 'qubodup' Gabovitch
+ Page: http://qubodup.net/
+ Origin: https://opengameart.org/content/impact
+ License: CC0