Add part13

part13/README.md

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+Part 12
+=======
+
+## Installation
+
+```bash
+elm-package install
+```
+
+(Answer `y` when prompted.)
+
+
+## Building
+
+```bash
+elm-live Main.elm --open --pushstate --output=elm.js
+```
+
+## Compiling CSS
+
+```bash
+elm-css Stylesheets.elm
+```
+
+## References
+
+* [Elm CSS documentation](http://package.elm-lang.org/packages/rtfeldman/elm-css/latest)