add doco for installing with composer

Signed-off-by: Joshua Paling <joshua.paling@gmail.com>
Merge pull request #162 from joshuapaling/doco-composer

add doco for installing with composer
Merge pull request #156 from joshuapaling/master

fix 'Undefined index: id' errors in isFileUploadOrHasExistingValue when ...
Release 1.0.0
Move plugin repository to cakephp-upload