Lightweight alternative to Ionide

[r734]

Thanks for this great F# teaching repo! It's really helping me to get used to F#'s unique syntax.

The only recommendation I have is to consider mentioning George Fraser's F# Language Server (https://github.com/georgewfraser/fsharp-language-server) as a lightweight replacement for Ionide in your readme. It doesn't require any setup beyond having .NET Core installed, and it generally stays out of my way and runs more smoothly.