Add docs about referencing local plugins

Author: rrrene

guides/plugins/creating_plugins.md

@@ -1,6 +1,8 @@
 # Creating Plugins
 
-A plugin is basically just a module that provides an `init/1` callback, taking a `Credo.Execution` struct as its only parameter.
+## Basics
+
+A plugin is basically just a module that provides an `init/1` callback, taking a `Credo.Execution` struct as its only parameter and returning a `Credo.Execution`. That's basically it.
 
 ```elixir
 defmodule CredoDemoPlugin do
@@ -40,7 +42,43 @@ You can find more information on `Credo.Plugin` and the functions imported:
 - `Credo.Plugin.prepend_task/3`
 - `Credo.Plugin.prepend_task/4`
 
+## Development
+
+Plugins are generally developed by putting them in a Hex package, referencing that package in `mix.exs` and then configuring the plugin in `.credo.exs`.
+
+However, for local development it can be beneficial to develop a plugin inside a project.
+But referencing modules from the current Mix project in `.credo.exs` does not work out of the box, because the project is not loaded for every `mix` task.
+
+To be able to use a module from the current project in `.credo.exs`, run `mix app.config` first:
+
+```bash
+mix do app.config + credo
+```
+
+This way, local plugins can be referenced in `.credo.exs`.
+
+Another, even more pragmatic way to do it is to run `app.config` from `.credo.exs` directly (since it is just a script file):
+
+```elixir
+Mix.Task.run("app.config", [])
+
+%{
+  configs: [
+    %{
+      name: "default",
+      plugins: [
+        {MyProject.CredoPlugin, []}
+      ]
+    }
+  ]
+}
+```
+
+This should naturally taken with a grain of salt, e.g. taking steps that this is only active during development.
+
+## Further reading
+
 The demo plugin used in the docs can be found on GitHub and Hex:
 
 - https://github.com/rrrene/credo_demo_plugin
-- https://hex.pm/packages/credo_demo_plugin
+- https://hex.pm/packages/credo_demo_plugin