Added PyPi installation instructions

Author: evaherrada

README.rst

@@ -52,6 +52,31 @@ Please ensure all dependencies are available on the CircuitPython filesystem.
 This is easily achieved by downloading
 `the Adafruit library and driver bundle <https://github.com/adafruit/Adafruit_CircuitPython_Bundle>`_.
 
+Installing from PyPI
+====================
+
+On supported GNU/Linux systems like the Raspberry Pi, you can install the driver locally `from
+PyPI <https://pypi.org/project/adafruit-circuitpython-ds3231/>`_. To install for current user:
+
+.. code-block:: shell
+
+    pip3 install adafruit-circuitpython-ds3231
+
+To install system-wide (this may be required in some cases):
+
+.. code-block:: shell
+
+    sudo pip3 install adafruit-circuitpython-ds3231
+
+To install in a virtual environment in your current project:
+
+.. code-block:: shell
+
+    mkdir project-name && cd project-name
+    python3 -m venv .env
+    source .env/bin/activate
+    pip3 install adafruit-circuitpython-ds3231
+
 Usage Notes
 ===========