Add contributing and developers guide documentation

[djhoese]

This adds a contributing page and basic developer's guide to the sphinx documentation. The contributing file actually lives in the root of the repository so that github can find it and suggest contributors read it before submitting pull requests.

This also includes a fix to one of the new helper functions so sphinx can build it.

• Fully documented

[CLAassistant]

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[coveralls]

Coverage remained the same at 57.57% when pulling fdf5133 on doc-contributing into 58d2ec8 on develop.

[mraspaud]

LGTM