Add 'rake db:create' instruction

Add an instruction to create the database which better reflects reality, 
especially when going through the 'Quick Install' shown on the first page
of the Padrino website.

    gem install padrino
    padrino g project myapp -d datamapper -b
    cd myapp
    padrino g admin
    padrino rake dm:migrate seed
    padrino start