Document the required language extensions

[sjakobi]

These seem to be DataKinds and TypeOperators for defining functions with named parameters, and OverloadedLabels for using them. The last one admittedly took me a bit to figure out – I wasn't sure what to search for.

EDIT: The error when you haven't enabled OverloadedLabels also isn't helpful at all:

> :t createSymLink ! #from "/path/to/source"

<interactive>:1:17: error: parse error on input ‘#’

[adamgundry]

I've brought this up on https://gitlab.haskell.org/ghc/ghc/-/issues/10998#note_298525 - it would be nice if the GHC parse error suggested enabling OverloadedLabels.