readme: recommend tmpfs for file cache
readme: recommend tmpfs for file cache

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