(#17945) Sync README.md with content on puppet-docs around PG SSL

As my next commit will involve modifying the SSL specific PostgreSQL docs
this step simply syncs up that section with what is on puppet-docs first.

Signed-off-by: Ken Barber <ken@bob.sh>
(#17945) Expand the documentation for PostgreSQL SSL client configuration

Previously we had just linked the user off to the PostgreSQL website for the
info. This attempts to break down the 3 methodologies but without stepping
too far into JDBC specific setup instructions. This avoids repeating
the information provided already on the PostgreSQL JDBC site, but at least
gives the intended user some idea of what high-level steps are required
depending on their desired setup.

Signed-off-by: Ken Barber <ken@bob.sh>