Add warning to not use

README.md

@@ -1,6 +1,8 @@
 FFI
 ===
 
+*** Warning: Turns out this is a really bad idea, do not use! When the BEAM calls native functions it needs to know how many reductions should be used. Otherwise calling a long running function can break the scheduling completly, because with this approach a long running function is considered 1 reduction ***
+
 An easy way to call external functions (e.g. C functions) from [Elixir](https://github.com/elixir-lang/elixir).
 
 Inspired by [Ruby's FFI](https://github.com/ffi/ffi)