(#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) Use postgresql 9.2 jdbc

This is required so we can use the LibPQFactory provided with 9.2. This gives
us the capability to specify pem files for the CA/root certificate for SSL
connectivity with PostgreSQL.

Notice we are not using the postgresql groupId, but are instead using a version
we have published to clojars in the puppetlabs namespace:

https://clojars.org/puppetlabs/postgresql

This is due to the fact that at the time of this patch, the latest JDBC driver
had not been published to the central maven repo:

https://groups.google.com/forum/?fromgroups=#!searchin/pgsql.interfaces.jdbc/maven/pgsql.interfaces.jdbc/ctUBlGTmoT8/3ouh6HGNFz0J

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