improve FSI error message about extensions

[smoothdeveloper]

Currently

#r "csc.exe: MyCode.cs";;

error FS3216: Package manager key 'csc.exe' was not registered in [C:\Program Files\dotnet\sdk\5.0.100\FSharp; C:\Program Files\dotnet\sdk\5.0.100\FSharp], []. Currently registered: nuget, paket

Proposing

I would like the error message to point at the readme.md.

error FS3216: Package manager key 'csc.exe' was not registered in [C:\Program Files\dotnet\sdk\5.0.100\FSharp; C:\Program Files\dotnet\sdk\5.0.100\FSharp], []. Currently registered: nuget, paket.
To learn more about extensions, visit: https://aka.ms/dotnetdepmanager

[dsyme]

@smoothdeveloper Could you submit a PR for this small improvement?

[smoothdeveloper]

@dsyme yes I would like to improve it, can we define a stable URI?

Also, I'd like some content I made in this PR to be salvaged, if it is useful to put on such page:

#10511