Add src/include/catalog/README

This just includes a link to the bki documentation, to help people get
started.

Before commit 372728b0d4, 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

src/include/catalog/README

@@ -0,0 +1,2 @@
+See <https://www.postgresql.org/docs/devel/bki.html> about the
+files in this directory.