Add doc for haskell.cabalFormattingProvider

Author: jian-lin

docs/configuration.md

@@ -41,6 +41,7 @@ This option obviously would not make sense for language servers for other langua
 Here is a list of the additional settings currently supported by `haskell-language-server`, along with their setting key (you may not need to know this) and default:
 
 - Formatting provider (`haskell.formattingProvider`, default `ormolu`): what formatter to use; one of `floskell`, `ormolu`, `fourmolu`, or `stylish-haskell`.
+- Cabal formatting provider (`haskell.cabalFormattingProvider`, default `cabal-gild`): what formatter to use for cabal files; one of `cabal-gild` or `cabal-fmt`.
 - Max completions (`haskell.maxCompletions`, default 40): maximum number of completions sent to the LSP client.
 - Check project (`haskell.checkProject`, default true): whether to typecheck the entire project on initial load. As it is activated by default could drive to bad performance in large projects.
 - Check parents (`haskell.checkParents`, default `CheckOnSave`): when to typecheck reverse dependencies of a file; one of `NeverCheck`, `CheckOnSave` (means dependent/parent modules will only be checked when you save), or `AlwaysCheck` (means re-typechecking them on every change).