Add README

Signed-off-by: Mark Hymers <mark@hymers.org.uk>

README.md

+# PiN Course Material
+
+This directory contains material for the PiN course and assessments.
+
+Students should clone this repository:
+
+```
+git clone https://vcs.ynic.york.ac.uk/cn/pin-material
+```
+
+New material will be added during the course, so you should regularly
+refresh your copy by changing into the directory and doing a git pull:
+
+```
+cd pin-material
+git pull
+```