updated readme

Readme.md

@@ -6,7 +6,8 @@
 that provides mechanisms for manipulating HTTP cache headers according to
 [RFC 2616 §14.9](https://tools.ietf.org/html/rfc2616#section-14.9).
 
-Supports Fastify versions ^2.0.0.
+Supports Fastify versions ^3.0.0. Version v5.x supports Fastify ^3.0.0 in
+the [`v5.x` branch](https://github.com/fastify/fastify-caching/tree/v5.x).
 
 This plugin fully supports Fastify's encapsulation. Therefore, routes that
 should have differing cache settings should be registered within different