Add a note about dropped Python 2 support

README.md

@@ -11,9 +11,11 @@ documents from reStructuredText files.
 This repository contains a Python package `docutils_extended` that can be installed with `pip`:
 
 ```bash
-pip install git+https://iffgit.fz-juelich.de/doc-utils/doc-utils
+python3 -m pip install git+https://iffgit.fz-juelich.de/doc-utils/doc-utils
 ```
 
+(Starting with version `v0.4.0` Python 2 is no longer supported.)
+
 After installing you can use the extended rst writers `rst2latex-extended` and `rst2html-extended` that are compatible
 with the standard versions but offer additional features: