Re-add part3

advanced/part3/README.md

@@ -0,0 +1,13 @@
+# Part 3
+
+To build everything, `cd` into this `part3/` directory and run:
+
+```shell
+elm make src/Main.elm --output=../server/public/elm.js
+```
+
+Then open [http://localhost:3000](http://localhost:3000) in your browser.
+
+## Exercise
+
+Open `src/Page/Article.elm` in your editor and resolve the TODOs there.