Add installation docs page.

docs/installation.md

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+# Installation
+
+This section explains how to install `django-commons-playground` to use within
+your project. It assumes you have a working installation of Python 3.
+
+1. Create venv/virtualenv (`python -m venv venv`)
+2. Activate venv/virtualenv (`source venv/bin/activate`)
+3. Install `django-commons-playground` (`pip install django-commons-playground`)
+
+All together now:
+
+```shell
+python -m venv venv
+source venv/bin/activate
+pip install django-commons-playground
+```