Add part5

intro/part5/README.md

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+# Part 5
+
+This time, we'll still be *building* `src/Main.elm`, but a few things will be different:
+
+1. We'll be *editing* a different file. (`elm make` will figure out it needs to compile our edited file as well, because `src/Main.elm` imports it!)
+2. We'll open `localhost:3000` instead of `index.html`.
+3. We'll specify a different `--output` target. (Explained next.)
+
+To build everything, `cd` into the `part5/` directory and run:
+
+```shell
+elm make src/Main.elm --output ../server/public/elm.js
+```
+
+Then open `http://localhost:3000/#/register` in your browser. (Opening `index.html` will not work anymore; from now on we'll be using the server we set up at the beginning of the workshop!)
+
+## Exercise
+
+Open `src/Page/Register.elm` in your editor and resolve the TODO there.
+
+This time we'll be fixing a bug in an existing code base! It's only one TODO,
+so that you have time to orient yourself in an unfamiliar code base.
+
+Because this is a more real-world code base, it uses some concepts we haven't
+covered yet. For example, you may be wondering things like "What does Cmd.none
+do?" This is okay! You won't need to know those concepts to complete the exercise.
+
+You may surprise yourself at how well you can already navigate around an Elm
+code base, despite not knowing 100% of what the code is doing. As you'll
+see, the compiler has your back!
+
+