Added the generated design docs.

Also explained how to generate them in the README.md file.

README.md

@@ -93,8 +93,33 @@ To build and flash the program to your Microbit just run:
 cargo embed
 ```
 
+## Documentation
 
+### Design Doc
 
+The [Design Doc][./docs/design.pdf] is generated with pandoc from the markdown
+file.
+
+```bash
+   pandoc ./docs/design.md -o ./docs/design.pdf
+```
+
+### Software Design Doc
+
+The [Software Design Doc][./docs/sdd.pdf] is generated with pandoc from the markdown
+file.
+
+```bash
+   pandoc ./docs/sdd.md -o ./docs/sdd.pdf
+```
+
+### UML
+
+[UML][./docs/uml/] is generated using Plant UML on the \*.puml files.
+
+```bash
+   plantuml ./docs/uml/*.puml
+```
 ---
 
 ## Copyright & License