update the readme.md file with installation instructions

README.md

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 Development of a low-cost multi-electrodes resistivimeter  for electrical resistivity mesaurement for lab applications, based on raspberry board
 
 Version 1.00
+
+## Installation
+
+All dependencies are specified in *requirements.txt*
+
+**Note:** all instructions below should be typed in the terminal
+
+It is first necessary to ensure that the libatlas-base-dev library is installed:
+`sudo apt-get install libatlas-base-dev`
+
+We strongly recommend users to create a virtual environment to run the code and installed all required dependencies. It can be done either in a directory gathering all virtual environments used on the computer or within the ohmpy directory.
+*  create the virtual environment:
+   `python3 -m venv ohmpy`
+*  activate it using the following command:
+   `source ohmpy/bin/activate`
+*  install packages within the virtual environment. Installing the following package should be sufficient to meet dependencies:
+`pip install RPi.GPIO adafruit-blinka numpy adafruit-circuitpython-ads1x15 pandas`
+*  check that requirements are met using `pip list`
+*  you should run you code within the virtual environment
+*  to leave the virtual environment simply type: `deactivate`
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