Added exercise to application structure section
Added exercise to application structure section

girldevelopit/gdi-intro-php-mysql,Roenok/gdi-intro-php-mysql,jessappeldoorn/gdi-intro-php-mysql,ellotheth/gdi-intro-php-mysql,pamcez/gdi-intro-php-mysql,girldevelopit/gdi-intro-php-mysql,jessappeldoorn/gdi-intro-php-mysql,ellotheth/gdi-intro-php-mysql,Roenok/gdi-intro-php-mysql,pamcez/gdi-intro-php-mysql