fix readme

sandbox.lua

@@ -0,0 +1,10 @@
+function sandbox(func)
+    local co = coroutine.create(func)
+    local hook = function() coroutine.yield('resource used too many cycles') end
+
+    debug.setupvalue(func, 1, _G)
+    debug.sethook(co, hook, "", 100)
+
+    coroutine.resume(co)
+end
+