Add link to Elm proto plugin

docs/third_party.md

@@ -32,6 +32,7 @@ These are projects we know about implementing Protocol Buffers for other program
 * Delphi: http://sourceforge.net/projects/protobuf-delphi/
 * Delphi: http://fundementals.sourceforge.net/dl.html
 * Elixir: https://github.com/jeremyong/exprotoc
+* Elm: https://github.com/tiziano88/elm-protobuf
 * Erlang: http://github.com/ngerakines/erlang_protobuffs/tree/master
 * Erlang: http://piqi.org/
 * Erlang: https://code.google.com/p/protoc-gen-erl/