Update cache_expiration.conf

Added explanatory text in case "ExpiresDefault" is not applied by default.

h5bp/web_performance/cache_expiration.conf

@@ -48,6 +48,13 @@
     ExpiresByType application/xml                       "access"
     ExpiresByType text/xml                              "access"
 
+# Keep in mind that the webserver config may have already a preset some file types.
+# If so the general rule with "ExpiresDefault" won't be applied for these types.
+# This applies especially to image types. If you are not sure or are not able to check the config
+# you can test your website with tools such as "Google Lighthouse" or https://pagespeed.web.dev/ 
+# and look for complaints like "Serve static assets with an efficient cache policy".
+# In order to override any preset(s), uncomment the appropriate lines below.
+
     # Generic: WebAssembly
     # ExpiresByType application/wasm                    "access plus 1 year" # default