docs: add in Mill instructions

I recently made a Mill plugin for Giter8 so that Mill users can also
have the benefit of being able to test the generation of their
templates. I added some small notes and instructions on how to use it
under "Testing templates locally", but let me know if there is somewhere
else that is more fitting. Also if you'd like to not include external
plugins in the official docs, then no problem, feel free to close.

docs/03/b.md

@@ -66,5 +66,28 @@ project with a `.g8` extension) you can test it with the actual g8
 runtime. When you're ready, add your template project to the
 [the wiki][wiki] so other giter8 users can find it.
 
+### Using Mill
+
+There is also an external [Mill plugin][mill-plugin] that can be used to test
+your templates as well. An example setup can be found below:
+
+```scala
+import $ivy.`io.chris-kipp::mill-giter8::0.2.0`
+
+import io.kipp.mill.giter8.G8Module
+
+object g8 extends G8Module {
+  override def validationTargets =
+    Seq("example.compile", "example.fix", "example.reformat")
+}
+```
+
+This plugin only supprts [`src` layouts][src-layout], but gives you some useful
+targets like `g8.validate` which will both test the generation of your template
+and also ensure any targets defined with `validationTargets` can also be ran
+against your generated project.
+
   [scripted]: https://www.scala-sbt.org/1.x/docs/Testing-sbt-plugins.html
   [wiki]: https://github.com/foundweekends/giter8/wiki/giter8-templates
+  [mill-plugin]: https://github.com/ckipp01/mill-giter8
+  [src-layout]: http://www.foundweekends.org/giter8/template.html#template+layout