The GNU readline library is now an INTERNALLIB - that is, it is
statically linked into consumers (GDB and variants) in the base
system, and the shared library is no longer installed.

That also allows ports to be able to use a modern version of readline

PR:		162948
Reviewed by:	emaste
Document r268461, readline(3) shlib no longer installed, and
statically linked into consumers in the base system.

Sponsored by:	The FreeBSD Foundation