[chores] Added ReadTheDocs configuration #211

[pandafy]

Closes #211

[pandafy]

The ReadTheDocs build is passing for this PR: https://readthedocs.org/projects/netjsonconfig/builds/16487366/

[coveralls]

Coverage remained the same at 99.604% when pulling 6f01968 on issues/211-read-the-docs into 760911c on master.

[nemesifier]

Still failing:

Running Sphinx v1.8.6
loading translations [en]... done
making output directory...
loading pickled environment... done
building [mo]: targets for 0 po files that are out of date
building [latex]: all documents
updating environment: 0 added, 0 changed, 0 removed
looking for now-outdated files... none found
processing netjsonconfig.tex...index general/setup general/basics backends/openwrt backends/openwisp backends/vpn backends/openvpn backends/wireguard backends/vxlan_over_wireguard backends/create_your_backend general/commandline_utility general/running_tests general/contributing general/goals general/changelog 
resolving references...
/home/docs/checkouts/readthedocs.org/user_builds/netjsonconfig/envs/issues-211-read-the-docs/lib/python3.8/site-packages/sphinx/builders/latex/__init__.py:201: FutureWarning: 
   The iterable returned by Node.traverse()
   will become an iterator instead of a list in Docutils > 0.16.
  if toctrees[0].get('maxdepth') > 0:
/home/docs/checkouts/readthedocs.org/user_builds/netjsonconfig/checkouts/issues-211-read-the-docs/docs/source/index.rst:: WARNING: a suitable image for latex builder not found: ['image/svg+xml'] (https://travis-ci.org/openwisp/netjsonconfig.svg)
/home/docs/checkouts/readthedocs.org/user_builds/netjsonconfig/checkouts/issues-211-read-the-docs/docs/source/index.rst:: WARNING: a suitable image for latex builder not found: ['image/svg+xml'] (https://coveralls.io/repos/openwisp/netjsonconfig/badge.svg)
/home/docs/checkouts/readthedocs.org/user_builds/netjsonconfig/checkouts/issues-211-read-the-docs/docs/source/index.rst:: WARNING: a suitable image for latex builder not found: ['image/svg+xml'] (https://img.shields.io/librariesio/release/github/openwisp/netjsonconfig)
/home/docs/checkouts/readthedocs.org/user_builds/netjsonconfig/checkouts/issues-211-read-the-docs/docs/source/index.rst:: WARNING: a suitable image for latex builder not found: ['image/svg+xml'] (https://badge.fury.io/py/netjsonconfig.svg)
/home/docs/checkouts/readthedocs.org/user_builds/netjsonconfig/checkouts/issues-211-read-the-docs/docs/source/index.rst:: WARNING: a suitable image for latex builder not found: ['image/svg+xml'] (https://img.shields.io/gitter/room/nwjs/nw.js.svg?style=flat-square)
writing... 
Markup is unsupported in LaTeX:
backends/wireguard:: nested tables are not yet implemented.

[nemesifier]

Let's try it one more time.