Update readme to mention fastify 4.x (#209)

I'm trying to use this package with fastify 4.0.2, and it seems like the latest 8.x version supports it.  But, I haven't had any luck yet and maybe the rest of the docs are out of date.  But at least I think this change is accurate.

README.md

@@ -7,9 +7,11 @@
 
 `@fastify/cors` enables the use of [CORS](https://en.wikipedia.org/wiki/Cross-origin_resource_sharing) in a Fastify application.
 
-Supports Fastify versions `3.x`.
-Please refer to [this branch](https://github.com/fastify/fastify-cors/tree/3.x) and related versions for Fastify `^2.x` compatibility.
-Please refer to [this tag](https://github.com/fastify/fastify-cors/tree/v1.0.0) and related versions for Fastify `^1.x` compatibility.
+Supports Fastify versions `4.x`.
+
+- Please refer to [7.x](https://github.com/fastify/fastify-cors/tree/v7.0.0) for Fastify `^3.x` compatibility.
+- Please refer to [3.x](https://github.com/fastify/fastify-cors/tree/3.x) for Fastify `^2.x` compatibility.
+- Please refer to [1.x](https://github.com/fastify/fastify-cors/tree/v1.0.0) for Fastify `^1.x` compatibility.
 
 ## Install
 ```