Add part3

intro/part3/README.md

@@ -0,0 +1,21 @@
+# Part 3
+
+This is just like last time, except first we'll install the `elm/browser` package so we can create an interactive app in the browser, instead of static HTML.
+
+To install the package, `cd` into the `part3/` directory and run:
+
+```shell
+elm install elm/browser
+```
+
+Then build everything the same way as last time:
+
+```shell
+elm make src/Main.elm --output elm.js
+```
+
+Finally, open `index.html` in your browser.
+
+## Exercise
+
+Open `src/Main.elm` in your editor and resolve the TODOs there.