Add on save usage section to elm layer

According to official elm-format documentation there is elm-format-on-save option. I've added it to layer documentation and introduced "On save usage" section which now contains info about elm format on save and imports sort on save.

layers/+lang/elm/README.org

@@ -15,7 +15,9 @@
  - [[#basic-usage-tips][Basic usage tips]]
    - [[#compilation][Compilation]]
    - [[#reactor][Reactor]]
-   - [[#sort-imports-on-save][Sort imports on save]]
+   - [[#on-save-usage][On save usage]]
+     - [[#imports-sort][Imports sort]]
+     - [[#file-format][File format]]
    - [[#indentation][Indentation]]
  - [[#key-bindings][Key bindings]]
    - [[#elm-make][elm-make]]
@@ -137,7 +139,8 @@ 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
+** On save usage
+*** Imports sort
 Set ~elm-sort-imports-on-save~ to ~t~ to sort the imports in the current file on
 every save.
 
@@ -145,6 +148,13 @@ every save.
   (elm :variables elm-sort-imports-on-save t)
 #+END_SRC
 
+*** File format
+Set ~elm-format-on-save~ to ~t~ to format current file on every save.
+
+#+BEGIN_SRC emacs-lisp
+  (elm :variables elm-format-on-save t)
+#+END_SRC
+
 ** Indentation
 elm-mode indentation is based on cycling: every time you insert a new line there
 will be one or more indentation levels available for you to choose from. The