improve composer doco - link to packagist & more.
improve composer doco - link to packagist & more.

Signed-off-by: Joshua Paling <7176df467bdfdaea708047a9df6afb8c26fa3734@gmail.com>

josegonzalez/cakephp-upload,zynesis/cakephp-upload,ramiroaraujo/cakephp-upload,Phillaf/cakephp-upload,silviolleite/cakephp-upload,amstegraf/cakephp-upload