elm: mention elm-sort-imports-on-save in documentation

layers/+lang/elm/README.org

@@ -14,6 +14,7 @@
  - [[#basic-usage-tips][Basic usage tips]]
    - [[#compilation][Compilation]]
    - [[#reactor][Reactor]]
+   - [[#sort-imports-on-save][Sort imports on save]]
  - [[#key-bindings][Key bindings]]
    - [[#elm-make][elm-make]]
    - [[#elm-repl][elm-repl]]
@@ -114,6 +115,14 @@ can be controlled by passing in these variables in your =~/.spacemacs=:
        elm-reactor-address "0.0.0.0") ; default 127.0.0.1
 #+END_SRC
 
+** Sort imports on save
+Set ~elm-sort-imports-on-save~ to ~t~ to sort the imports in the current file on
+every save.
+
+#+BEGIN_SRC emacs-lisp
+  (elm :variables elm-sort-imports-on-save t)
+#+END_SRC
+
 * Key bindings
 ** elm-make