Contributing.rst documentation is failing on make html

[mzyndul]

Minimal, reproducible code sample, a copy-pastable example if possible

Documentation is built with `sphinx <http://sphinx-doc.org/>`_. To build::

    $ pip install sphinx sphinx_issues
    $ cd docs
    $ make html

Problem description

Creating docs doesn't work out of the box as described in documentation:

➜ make html                  
sphinx-build -b html -d _build/doctrees   . _build/html
Running Sphinx v3.5.4

Extension error:
Could not import extension sphinx_issues (exception: No module named 'sphinx_issues')
make: *** [Makefile:40: html] Error 2

Since the problem is quite obvious, documentation should mention about missing package sphinx_issues, I would see one of the following as the solution:

pip install sphinx sphinx_issues

OR

pip install -r requirements-docs.txt

while I think using requirements file is better.

I could create PR for this.

[juarezr]

Oh! Please, go ahead and make your PR.

I think this code path needs some love. :)
Also, there is a way for building docs using tox.

I'm glad to help if you need any help.

[mzyndul]

PR merged, I close it.

[juarezr]

Ok, thanks.
If you have other improvements, please open a new PR.