Add readme file

A readme including basic ansible knowledge to install a system and to
maintain this repo

README.md

+# Ansible repository for LAS/CS NSQ computer
+`git clone git@git.scc.kit.edu:las/ansible.git`
+
+## How to use ansible for software installation/computer setup
+Add your computer to the hosts file
+install ansible
+run ansible-pull
+
+### Ask for new software
+Open an issue in the GitLab issue tracker with the label: softwarerequest
+## How to get new software on your computer
+
+## Available roles
+
+* common.yml: basic configuration for all LAS/NSQ computers
+* clients.yml: all computers not acting as a server (only)
+* desktop.yml: all desktop computers including laptops (having X11/Wayland)
+* admin.yml: tools for administrators
+
+# Develope new roles, extend or modify existing ones and update roles for new software
+
+## Branches
+All roles in the master branch should work and should not brake on any of our systems (desktop, server, simulation, notebooks) and the site.yml should always be runnable.
+
+For developement and testing you should use development branches
+