documentation: clarify db permissions in getting started doc
documentation: clarify db permissions in getting started doc

In the 'getting started' doc create a user and database for dex to
clarify that the user needs permissions to create databases, but
dex does not.

closes #127

andrewstuart/dex,bcwaldon/dex,sercand/dex,zlabjp/dex,bcwaldon/dex,andrewstuart/dex,sercand/dex,Tecsisa/dex,Tecsisa/dex,andrewstuart/dex,zlabjp/dex,sercand/dex,jhuntoo/dex,jhuntoo/dex,zlabjp/dex,jhuntoo/dex,bcwaldon/dex,Tecsisa/dex