Updated quickstart documentation
Updated quickstart documentation

Added a line stating that clients are immutable
paveljanda/guzzle,thinsoldier/guzzle,guzzle/guzzle,Tobion/guzzle,dshafik/guzzle,Brunty/guzzle,michabbb-backup/guzzle,Tobion/guzzle,ehough/guzzle,thinsoldier/guzzle,mortenhauberg/guzzle,kkopachev/guzzle,ehough/guzzle,kkopachev/guzzle,Brunty/guzzle,dshafik/guzzle,wherefortravel/guzzle,paveljanda/guzzle,wherefortravel/guzzle,michabbb-backup/guzzle,mortenhauberg/guzzle,guzzle/guzzle