docs(readme): update plugin version syntax (#198)

Author: Fdawgs

README.md

@@ -15,11 +15,11 @@ npm i pg @fastify/postgres
 ### Compatibility
 | Plugin version | Fastify version |
 | ---------------|-----------------|
-| `^6.x`         | `^5.x`          |
+| `>=6.x`        | `^5.x`          |
 | `^5.x`         | `^4.x`          |
-| `^3.x`         | `^3.x`          |
-| `^1.x`         | `^2.x`          |
-| `^1.x`         | `^1.x`          |
+| `>=3.x <5.x`   | `^3.x`          |
+| `>=1.x <3.x`   | `^2.x`          |
+| `>=1.x <3.x`   | `^1.x`          |
 
 
 Please note that if a Fastify version is out of support, then so are the corresponding versions of this plugin