List what specific features we use from the F# language

[nojaf]

When we asked people in the past if they want to fix their own issue, we something got the reply: "No, because I'm new to F#".

That actually shouldn't be a showstopper. We should list the language features that we often use and try to give some pointers for those newcomers on how to get up to speed.

A couple of topics:

• Custom operators: https://docs.microsoft.com/en-us/dotnet/fsharp/language-reference/operator-overloading#creating-new-operators
• Active patterns: https://docs.microsoft.com/en-us/dotnet/fsharp/language-reference/active-patterns
• Signature files: https://docs.microsoft.com/en-us/dotnet/fsharp/language-reference/signature-files

[nojaf]

This has been addressed, see https://fsprojects.github.io/fantomas/docs/contributors/FSharp.html