Add readme

README.md

+This is a public repository that contains a collection of our current (and past) LinuxDays courses.
+
+# For Course Attendants
+
+If you are a course attendant, you can clone (download) this repository by issuing the following command in a terminal:
+
+```
+git clone https://gitlab.ethz.ch/thealternative/courses.git
+```
+
+You can find the course materials in the various subdirectories of this repository.
+
+# For TheAlternative Members
+
+This repository is kept up to date by TheAlternative members. If you teach a course, please add your slides and supplementary material, such as exercise sheets, to this repo. Do not upload any private files, as this repository is publicly visible.