added README with git instructions

README

+This is the Git repository containing the material for the course on Numerical Optimization Software.
+
+The easiest way to download files as soon as they are made available is to use Git.
+
+In a terminal, type
+$ git clone http://gitlab.syscop.de/teaching/NOS_public.git
+to clone the repository (only the first time).
+
+Afterwards, to download only the new commits, from the NOS_public folder type on a terminal
+$ git pull origin master
+
+This should be enough for the needs of the course.