Reconcile GitHub Wiki and `contributor_guide.md`

[HEdingfield]

Options:

• Move contributor_guide.md info into the GitHub Wiki (e.g. here) and link to the Wiki from the .md
• Write a quick summary in the Wiki here and link to contributor_guide.md

Maybe method 2 is preferable, since it makes us less dependent on GitHub?

[artoonie]

Not sure how expansive you were thinking -- is this sufficient? https://github.com/BrightSpots/rcv/wiki/Notes-on-Development

[HEdingfield]

LGTM. I guess as a final step it'd be good to link from contributor_guide.md back to that Wiki page as well, noting that there's additional useful info there.

[artoonie]

Done: https://github.com/BrightSpots/rcv/pull/841