Add CHANGELOG entry for ServerTiming middleware [ci skip]

Author: guilleiguaran

actionpack/CHANGELOG.md

@@ -1,3 +1,16 @@
+*  Add Server-Timing middleware
+
+   Server-Timing specification defines how the server can communicate to browsers performance metrics
+   about the request it is responding to.
+
+   The ServerTiming middleware is enabled by default on `development` environment by default using the
+   `config.server_timing` setting and set the relevant duration metrics in the `Server-Timing` header
+
+   The full specification for Server-Timing header can be found in: https://www.w3.org/TR/server-timing/#dfn-server-timing-header-field
+
+   *Sebastian Sogamoso*, *Guillermo Iguaran*
+
+
 ## Rails 7.0.0.alpha2 (September 15, 2021) ##
 
 *   No changes.