readme: recommend tmpfs for file cache
diff --git a/README.md b/README.md
index 7d66cf9..66f3d6a 100644
--- a/README.md
+++ b/README.md
@@ -72,7 +72,7 @@
 
 ```nginx
 pagespeed on;
-pagespeed FileCachePath /var/ngx_pagespeed_cache;
+pagespeed FileCachePath /var/ngx_pagespeed_cache;  # Use tmpfs for best results.
 ```
 
 In every server block where pagespeed is enabled add: