Add Elixir linter for dialyxir (#1257)

* Add Elixir linter for dialyxir

* Update doc/ale.txt with dialyxir

* Keep elixir tools alphabetically ordered in README

* Add a missing entry for dialyxir to the main documentation file.

doc/ale-elixir.txt

@@ -13,5 +13,20 @@ g:ale_elixir_mix_options                             *g:ale_elixir_mix_options*
 
   This variable can be changed to specify the mix executable.
 
+===============================================================================
+dialyxir                                                  *ale-elixir-dialyxir*
+
+Dialyzer, a DIscrepancy AnaLYZer for ERlang programs.
+http://erlang.org/doc/man/dialyzer.html
+
+It can be used with elixir through dialyxir
+https://github.com/jeremyjh/dialyxir
+
+Options for dialyzer are not configurable by ale, but they are instead
+configured on your project's `mix.exs`.
+
+See https://github.com/jeremyjh/dialyxir#with-explaining-stuff for more
+information.
+
 ===============================================================================
   vim:tw=78:ts=2:sts=2:sw=2:ft=help:norl:

doc/ale.txt

@@ -57,6 +57,7 @@ CONTENTS                                                         *ale-contents*
       hadolint............................|ale-dockerfile-hadolint|
     elixir................................|ale-elixir-options|
       mix.................................|ale-elixir-mix|
+      dialyxir............................|ale-elixir-dialyxir|
     elm...................................|ale-elm-options|
       elm-format..........................|ale-elm-elm-format|
       elm-make............................|ale-elm-elm-make|
@@ -301,7 +302,7 @@ Notes:
 * Dafny: `dafny`!!
 * Dart: `dartanalyzer`!!, `language_server`
 * Dockerfile: `hadolint`
-* Elixir: `credo`, `dogma`!!
+* Elixir: `credo`, `dialyxir`, `dogma`!!
 * Elm: `elm-format, elm-make`
 * Erb: `erb`, `erubis`
 * Erlang: `erlc`, `SyntaxErl`