Docs: More README Cleanup

Docs: More README Cleanup

Fix "Installing PostgreSQL server packages" heading.  It was showing
up as a list entry.

Drop the text about installing additional packages for the database,
since there are none to install at this time.  (We could replace that
with a subsection on the packages required for a standalong databse
server without all of Evergreen installed on it.)

Add a "Create the Evergreen PostgreSQL user" heading above the
instructions to creat the evergreen user in the database.

Co-authored-by: Ben Shum <ben@evergreener.net>
Signed-off-by: Jason Stephenson <jason@sigio.com>