expand README a bit

README.md

@@ -145,6 +145,10 @@ This requires setting the `TIMING_SECRET` CI variable.  Talk to an admin about
 this.  The variable should *not* be protected because we do timing measurements
 on all branches, not just `master`!
 
+Behind the scenes, the data is sent to our
+[custom webhook](https://gitlab.mpi-sws.org/janno/gitlab-ci-webhook), which runs
+on `coq-speed.mpi-sws.org` and locally stores the data in a PostgreSQL database.
+
 [coq-speed]: https://coq-speed.mpi-sws.org/
 
 ## Opam publishing