Update readme

README.md

@@ -1,4 +1,22 @@
-Ffi
+FFI
 ===
 
-** TODO: Add description **
+An easy way to call external functions (e.g. C functions) from Elixir.
+
+## Example Program
+
+```elixir
+defmodule MyLib do
+  use FFI.Library
+
+  ffi_lib "c"
+
+  attach_function :puts, [:string], :int
+end
+
+MyLib.puts(["Hello world"])
+```
+
+## License
+
+MIT