readme: clarify how to convert mod_pagespeed config to ngx_pagespeed
readme: clarify how to convert mod_pagespeed config to ngx_pagespeed

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