Update readme for new composer install.
Update readme for new composer install.

Composer require is the preferred method for installing packages.
twistor/flysystem,dshafik/flysystem,CarsonF/flysystem,rossriley/flysystem,oswaldderiemaecker/flysystem,nao-pon/flysystem,twistor/flysystem,sohelrana820/flysystem,mapyo/flysystem,mhlavac/flysystem,barryvdh/flysystem,dstockto/flysystem,sachintaware/flysystem,Amrit01/flysystem,mhlavac/flysystem,hannesvdvreken/flysystem,farmisen/flysystem