Added README describing mdBook setup

README.md

+XP-el Mikroprocessorprogrammeringskurs (MPPK)
+=============================================
+
+This uses [mdBook](https://github.com/rust-lang-nursery/mdBook) 
+Getting started:
+
+Install mdBook and the link-checker plugin:
+
+```
+cargo install mdbook-linkcheck
+```
+
+Run
+
+```
+mdbook serve
+```
+
+then open [http://localhost:3000](http://localhost:3000)
+
+and edit the files within `src`.