mix: add README (#7185)

plugins/mix/README.md

+# Mix plugin
+
+This plugin adds completions for the [Elixir's Mix build tool](https://hexdocs.pm/mix/Mix.html).
+
+To use it, add `mix` to the plugins array in your zshrc file:
+
+```zsh
+plugins=(... mix)
+```
+## Supported Task Types
+
+| Task Type               | Documentation                                            |
+|-------------------------|----------------------------------------------------------|
+| Elixir                  | [Elixir Lang](https://elixir-lang.org/)                  |
+| Phoenix v1.2.1 and below| [Phoenix](https://hexdocs.pm/phoenix/1.2.1/Phoenix.html) |
+| Phoenix v1.3.0 and above| [Phoenix](https://hexdocs.pm/phoenix/Phoenix.html)       |
+| Ecto                    | [Ecto](https://hexdocs.pm/ecto/Ecto.html)                |
+| Hex                     | [Hex](https://hex.pm/)                                   |