Updated documentation for PostgreSQL configuration to reflect changing

Hello, I've been doing the PuppetDB installation from source per the instructions and discovered that for new PostgreSQL users that the default authentication to the database is set to "ident" instead of "md5" which will not work by default with the current instructions.  I've added language to leave the default case currently alone but go on to explain that if it does not work how to change PostgreSQL to md5 from ident, how to restart the database, and retest login.  These directions worked for my install on CentOS 6 and should work for other users.