Update Slack URL and add Discord to template README

Author: jjcarstens

templates/new/README.md

@@ -28,5 +28,6 @@ To start your Nerves app:
   * Official docs: https://hexdocs.pm/nerves/getting-started.html
   * Official website: https://nerves-project.org/
   * Forum: https://elixirforum.com/c/nerves-forum
-  * Discussion Slack elixir-lang #nerves ([Invite](https://elixir-slackin.herokuapp.com/))
+  * Elixir Slack #nerves channel: https://elixir-slack.community/
+  * Elixir Discord #nerves channel: https://discord.gg/elixir
   * Source: https://github.com/nerves-project/nerves