Add explanatory text in case ExpiresDefault is not applied by default (#371)

Co-authored-by: Léo Colombaro <[email protected]>

Author: creopard

h5bp/web_performance/cache_expiration.conf

@@ -7,6 +7,13 @@
 # (!) If you don't control versioning with filename-based cache busting, you
 #     should consider lowering the cache times to something like one week.
 #
+# (!) When using `.htaccess` file, the webserver config may have already
+#     a preset some file types.
+#     In that case, the general rule with `ExpiresDefault` might not be applied.
+#     In order to override any presets, uncomment the appropriate "Generic"
+#     lines below.
+#     Online checker or validators can help investigating the served cache policy.
+#
 # https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/Cache-Control
 # https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/Expires
 # https://httpd.apache.org/docs/current/mod/mod_expires.html