Add installation and 'Hello World' chapter

This commit adds a chapter about the installation of python3, numpy, scipy and
matplotlib and a chapter with a first 'Hello World'-like example.

README.md

+This repository consists of a collection of python examples intended as an
+introduction on the usage of python in data analysis, especially for the
+advanced laboratories in physics at the University of Freiburg. In previous
+years code examples for [ROOT](https://root.cern.ch/) have been provided.
+Material on the usage of python was missing. The code examples shown in this
+repository follow the examples shown in the ROOT introductions.
+
+# Installation
+To get started with python for data analysis in the advanced laboratories you
+need the python interpreter. In this document we will use `python3`. The
+additional packages `numpy`, `scipy` and `matplotlib` are useful for data
+analysis and data presentation. To install all the packages on Ubuntu, you
+can run the following command line.
+
+
+```bash
+sudo apt-get install python3 python3-numpy python3-scipy python3-matplotlib
+```
+<!--
+Doxec in a docker container needs a slightly different command, please keep them
+in-sync.
+-->
+<!-- console
+```bash
+$ apt-get update
+$ apt-get install -y python3 python3-numpy python3-scipy python3-matplotlib
+```
+-->
+
+# 'Hello World' Example
+The first example is basically a 'Hello World' script, to check whether python
+is running correctly. Create a file named `hello_world.py` and add the following
+content.
+
+<!-- write hello_world.py -->
+```python
+import math
+
+print("Example 1:")
+print("  Square root of 2 = %g" % math.sqrt(2))
+```
+
+To run the example, open a terminal tell the python interpreter to run your
+code.
+<!-- console_output -->
+```sh
+$ python3 hello_world.py
+Example 1:
+  Square root of 2 = 1.41421
+```
+
+Have you seen the expected output? Congratulations, you can move on to real-life
+examples.