Doc: explain where to find Makefile used to build sepgsql-regtest.pp.

Doc: explain where to find Makefile used to build sepgsql-regtest.pp.

At least on Fedora and RHEL, it's not in the same RPM that's needed
for building sepgsql itself.  Today is the second or third time I've
had to rediscover how to install that, so let's document it this time.