RFQ CONTRIBUTING.rst for documentation

[jbqubit]

I propose modification to artiq/CONTRIBUTING.rst that emphasizes how to contribute documentation. At the ARTIQ Day it seemed clear there was interest from the community in improving documentation with emphasis on hangups encountered by relatively new users. To better support such contributions from such users a somewhat more explicit recipe is desirable to reduce the barrier to contribution. Here's a proposed edit.

Contributing Documentation

ARTIQ welcomes documentation contributions. A rendered html version of the ARTIQ manual is available here from the source doc/manual. Inline ARTIQ source documentation and the ARTIQ user manual use a variant of the reStructuredText (reST) markup language that is processed by the Sphinx Python documentation generation tool.

Write bite-sized patches that can stand alone, clean them up, write proper commit messages. Test that your edits for proper rendering prior to pushing upstream.

Instructions for testing documentation edits.

$ conda create -n sphinx sphinx sphinx-argparse sphinx_rtd_theme
$ conda activate sphinx
$ pip install sphinxcontrib-wavedrom
$ cd artiq/doc/manual
$ make html