how-to install for package dev docs

Author: najuzilu

docs/source/install.md

@@ -18,3 +18,14 @@ then run:
 ```bash
 python setup.py install
 ```
+
+## Package development
+
+To install `sphinxcontrib-prettyproof` for package development:
+
+```bash
+git clone https://github.com/najuzilu/sphinxcontrib-prettyproof
+cd sphinxcontrib-prettyproof
+git checkout master
+pip install -e .[all]
+```