Make note about Date header in masto compat

http-signature-normalization-actix/src/lib.rs

@@ -345,6 +345,9 @@ impl Config {
     /// Opt out of using the (created) and (expires) fields introduced in draft 11
     ///
     /// Use this for compatibility with mastodon
+    ///
+    /// Note that by enabling this, the Date header becomes required on requests. This is to
+    /// prevent replay attacks
     pub fn dont_use_created_field(self) -> Self {
         Config {
             config: self.config.dont_use_created_field(),