Update third_party.md

Add GDScript Protobuf implementation

docs/third_party.md

@@ -39,6 +39,7 @@ These are projects we know about implementing Protocol Buffers for other program
 * Erlang: https://code.google.com/p/protoc-gen-erl/
 * Erlang: https://github.com/basho/erlang_protobuffs
 * Erlang: https://github.com/tomas-abrahamsson/gpb
+* GDScript: https://github.com/oniksan/godobuf (Godot v3 engine plugin)
 * Go: https://github.com/golang/protobuf (Google-official implementation)
 * Go: https://github.com/akunspy/gopbuf
 * Go: https://github.com/gogo/protobuf