Add src/include/catalog/README
This just includes a link to the bki documentation, to help people get
started.
Before commit
372728b0d49, there was a README at
src/backend/catalog/README, but then this was moved to the SGML
documentation. So this effectively puts back a link to what was
moved. But src/include/catalog/ is probably a better location,
because that's where all the interesting files are.
Co-authored-by: Florents Tselai <florents.tselai@gmail.com>
Discussion: https://www.postgresql.org/message-id/flat/CA+v5N400GJFJ9RyXAX7hFKbtF7vVQGvWdFWEfcSQmvVhi9xfrA@mail.gmail.com