Add an Erlang project, gpb, to third_party.md

docs/third_party.md

@@ -36,6 +36,7 @@ These are projects we know about implementing Protocol Buffers for other program
 * Erlang: http://piqi.org/
 * Erlang: https://code.google.com/p/protoc-gen-erl/
 * Erlang: https://github.com/basho/erlang_protobuffs
+* Erlang: https://github.com/tomas-abrahamsson/gpb
 * Go: https://github.com/golang/protobuf (Google-official implementation)
 * Go: http://code.google.com/p/goprotobuf/
 * Go: https://github.com/akunspy/gopbuf