readme: keep it up to date with recent code changes, some rephrasing
readme: keep it up to date with recent code changes, some rephrasing

wanrui/ngx_pagespeed,digideskio/ngx_pagespeed,pagespeed/ngx_pagespeed,digideskio/ngx_pagespeed,wanrui/ngx_pagespeed,pagespeed/ngx_pagespeed,0x0mar/ngx_pagespeed,0x0mar/ngx_pagespeed