Update release steps (#116)

Use build is recommend now
https://packaging.python.org/en/latest/guides/distributing-packages-using-setuptools/#packaging-your-project

Co-authored-by: Manuel Kaufmann <humitos@gmail.com>

README.rst

@@ -3,7 +3,7 @@ Read the Docs Sphinx Extensions
 
 .. image:: https://img.shields.io/pypi/v/readthedocs-sphinx-ext.svg
    :target: https://pypi.python.org/pypi/readthedocs-sphinx-ext
-   :alt: Pypi Version 
+   :alt: PyPI Version
 .. image:: https://circleci.com/gh/readthedocs/readthedocs-sphinx-ext.svg?style=svg
    :target: https://circleci.com/gh/readthedocs/readthedocs-sphinx-ext
    :alt: Build Status
@@ -28,6 +28,7 @@ Releasing
 
     .. code:: bash
 
+        $ python -m pip install --upgrade pip build twine
         $ rm -rf dist/
-        $ python setup.py sdist bdist_wheel
+        $ python -m build --sdist --wheel
         $ twine upload --sign --identity security@readthedocs.org dist/*