Update license to CC-BY-SA-4.0

[kousu]

Our previous site/wiki was a DokuWiki site with content under CC-BY-SA-3.0. All content on the site that predates the switchover on June 4th last summer is covered by that. See neuropoly/neuro.polymtl.ca#73 (comment).

But all content added since then is in a legal grey zone, since we did not add a clear LICENSE at the time.

To clean up the situation, and also modernize the legal code, add LICENSE = CC-BY-SA-4.0 and a clear CONTRIBUTING.md that makes clear contributions will be taken under that license.

CC-BY-SA-3.0 is forward-compatible with CC-BY-SA-4.0, so this change does not need to involve content predating the switchover.

But everyone who has added content (current count: 22) since the switchover will need to give informed consent to this for this to be merged.

Fixes #68, if everyone consents.

(by the way, I got the bulk of the text from https://github.com/santisoler/cc-licenses)

This is the mirror of neuropoly/neuro.polymtl.ca#74

[elhil]

Closing as stale. Feel free to re-open if still relevant.