Its rivals seem to be protobuf and s-protobuf. The library documentation is included in the sources in a ReST document cl-protobufs.rst. You can git clone https://gitlab.common-lisp.net/qitab/cl-protobufs.git (If you're a developer, use git@gitlab.common-lisp.net/qitab/cl-protobufs.git). Or you can browse the git web interface at https://gitlab.common-lisp.net/qitab/cl-protobufs
Home page: http://common-lisp.net/project/qitab/
wire format