Added pbtools to third party implementations list.

docs/third_party.md

@@ -16,6 +16,7 @@ These are projects we know about implementing Protocol Buffers for other program
 * C: https://github.com/cloudwu/pbc/
 * C: https://github.com/haberman/upb/wiki
 * C: https://github.com/squidfunk/protobluff
+* C: https://github.com/eerimoq/pbtools
 * C++: https://github.com/google/protobuf (Google-official implementation)
 * C/C++: http://spbc.sf.net/
 * C#: http://code.google.com/p/protobuf-csharp-port