docs(readme): update plugin version syntax (#217)

Author: Fdawgs

README.md

@@ -23,9 +23,9 @@ npm i @fastify/request-context
 ### Compatibility
 | Plugin version | Fastify version |
 | ---------------|-----------------|
-| `^6.x`         | `^5.x`          |
-| `^4.x`         | `^4.x`          |
-| `^2.x`         | `^3.x`          |
+| `>=6.x`        | `^5.x`          |
+| `>=4.x <6.x`   | `^4.x`          |
+| `>=2.x <4.x`   | `^3.x`          |
 | `^1.x`         | `^2.x`          |
 | `^1.x`         | `^1.x`          |