Use creative commons share alike license for repository

[wiki-me]

I believe it is better for non code contributions (it is what wikipedia use) , the share alike variant (SA) acts like a copyleft license.

[Nutomic]

This is copied from the Rust RFC repo which is licensed under Apache 2.0 and MIT. I think it makes sense to use the same license as the code, as these are technical documents which are used directly together with it.