Update installation instruction

README.md

 # sense-hat
 
+## Optional virtual env
 
-## Utilisation
+`python3 -m venv sense-hat-env`
+
+`source sense-hat-env/bin/activate`
+
+
+## Installation
 
 `git clone https://gitlab.irstea.fr/raspberry-pi/sense-hat.git`
 
-`requirements.txt` TODO
+`sudo apt install libatlas-base-dev python3-sense-hat`
+
+`pip3 install -r sense-hat/requirements.txt`
+
+## Utilisation
 
-`python3 sense-hat/environmental_sensors.py`
\ No newline at end of file
+`python3 sense-hat/environmental_sensors.py`