add API documentation for modding extension

api.md

@@ -0,0 +1,21 @@
+# API Extension
+
+Adding your protections to the HUD
+----------------------------------
+
+If you are providing an extra protection mod to work in cunjunction with the
+HUD feature of `areas`, you can register a callback to add your mod's code to
+display your protection's existence. For example
+
+	local myhandler = function(pos,arealist)
+		local areaowner = find_my_protections(pos)
+
+		if areaowner then
+			arealist["mymodname"] = {
+				name = "Protection name",
+				owner = areaowner, 
+			}
+		end
+	end
+
+	areas.register_hud_handler(myhandler)