add stairs api (#56)

doc/stairs.md

@@ -0,0 +1,30 @@
+you can use this via `xcompat.stairs.register(nodename, def)`
+
+an example would be:  
+```lua
+xcompat.stairs.register(
+    "xcompat_stairs_test:fake_node",
+    core.registered_nodes["xcompat_stairs_test:fake_node"]
+)
+```
+
+if the game you are running on isnt supported (see readme), 
+it falls back to using a polyfill. each backend adds aliases 
+to the polyfill, mainly so that if we add a future backend 
+that ran on polyfill, everything keeps working (yay)
+
+at this time stairsplus/moreblocks compatibility/upgrading 
+isnt supported, however should be added in the future. for 
+now, in your mod code do something like the following:
+
+```lua
+if core.registered_modes("moreblocks") then
+    --call stairs plus
+else 
+    xcompat.stairs.register(node, def)
+end
+```
+
+that way in the future nothing will break when support is 
+added and at your convince the first part of the if can be 
+removed