add python setup instructions

README.md

+# Python setup
+In case a not installed python version is needed, follow these steps.
+
+## Download Python
+The required Python version can be downloaded from [here](https://www.python.org/downloads/).
+Extract it and open a terminal in the extracted folder.
+
+## Install Python
+Configure the installation.
+The `--with-openssl-rpath` is needed from Python 3.9 onwards, else pip will say it's missing ssl.
+```sh
+./configure --enable-optimizations --with-openssl-rpath=/usr/lib/x86_64-linux-gnu
+```
+
+Install Python
+```sh
+sudo make -j4 && sudo make altinstall
+```
+
+## Test
+If everything is set up correctly, the following command should work
+```sh
+pythonx.y -m ssl
+```
+
 # TensorFlow setup
 detailed instructions can be found at [tensorflow installation guide](https://www.tensorflow.org/install/pip), but they do not work out of the box!