Add v1.4.0 to Docs page (#916)

Author: eksperimental

docs.markdown

@@ -26,6 +26,15 @@ Choose which version you want documentation for.
 * [Logger](https://hexdocs.pm/logger/master/) - built-in Logger
 * [Mix](https://hexdocs.pm/mix/master/) - build tool
 
+<h4 id="v1.4">v1.4 <small>(<a href="https://github.com/elixir-lang/elixir/releases/download/v1.4.0/Docs.zip">download</a>)</small></h4>
+
+* [Elixir](https://hexdocs.pm/elixir/1.4.0/) - standard library
+* [EEx](https://hexdocs.pm/eex/1.4.0/) - templating library
+* [ExUnit](https://hexdocs.pm/ex_unit/1.4.0/) - unit test library
+* [IEx](https://hexdocs.pm/iex/1.4.0/) - interactive shell
+* [Logger](https://hexdocs.pm/logger/1.4.0/) - built-in Logger
+* [Mix](https://hexdocs.pm/mix/1.4.0/) - build tool
+
 <h4 id="v1.3">v1.3 <small>(<a href="https://github.com/elixir-lang/elixir/releases/download/v1.3.3/Docs.zip">download</a>)</small></h4>
 
 * [Elixir](https://hexdocs.pm/elixir/1.3.3/) - standard library