Add a note mentioning current .NET 7 trouble with Fable 3

Author: Maxime Mangel

docs/docs/2-steps/setup.md

@@ -7,6 +7,24 @@ layout: standard
 
 In order to use Fable you will need the following tools:
 
+:::warning
+Fable 3, currently has trouble with .NET 7.
+
+If you have .NET 7 installed on your system, you will need to add a `global.json` file to your project to ensure .NET 6 is used.
+
+*Example:* 
+
+`{
+    "sdk": {
+      "version": "6.0.0",
+      "rollForward": "latestMinor"
+    }
+}
+`
+
+You can also give a try to [Fable 4](https://fable.io/blog/2022/2022-09-28-fable-4-theta.html), as it already has .NET 7 support.
+:::
+
 - [.NET Core SDK](https://dotnet.microsoft.com/) to work with F# files and dependencies
 - [Node.js](https://nodejs.org/) to execute JS code
 - A JS package manager, like [npm](https://www.npmjs.com/) (which comes with Node) or [yarn](https://yarnpkg.com/)