docs: update installation guide in getting_started (#65)

Author: mvanwyk

docs/getting_started/installation.md

@@ -1 +1,16 @@
-## Coming Soon
+# Getting started
+
+PyRetailScience is a powerful framework for rapid bespoke and deep dive retail analytic. It equips you with a wide
+array of retail analytical capabilities, from segmentations to gain-loss analysis. Leave the mundane to us and elevate
+your role from data janitor to insights virtuoso.
+
+## Installation
+
+PyRetailScience is published as a [Python package](https://pypi.org/project/pyretailscience/) and can be installed with
+`pip`, ideally by using a
+[virtual environment](https://realpython.com/what-is-pip/#using-pip-in-a-python-virtual-environment). Open a terminal
+and install with:
+
+```bash
+pip install pyretailscience
+```