In mix task tailwind.install, add instructions for using custom binary

Currently, these instructions are in the README, but they do not appear on HexDocs.

This commit basically just copies the line from the README into the doc page for `mix tailwind.install` so that the information is available in both places.

Author: arcanemachine

lib/mix/tasks/tailwind.install.ex

@@ -10,6 +10,15 @@ defmodule Mix.Tasks.Tailwind.Install do
 
       config :tailwind, :version, "#{Tailwind.latest_version()}"
 
+  To install the Tailwind binary from a custom URL (e.g. if your platform isn't
+  officially supported by Tailwind), you can supply a third party path to the
+  binary (beware that we cannot guarantee the compatibility of any third party
+  executable):
+
+  ```bash
+  $ mix tailwind.install https://people.freebsd.org/~dch/pub/tailwind/v3.2.6/tailwindcss-freebsd-x64
+  ```
+
   ## Options
 
       * `--runtime-config` - load the runtime configuration