Document the YAML specification version and schema

[milliams]

I'm trying to find documented the version of the YAML spec that this library supports. From some issues here I've seen hinted that it seems to be YAML 1.2 from 2009 (presumably 1.2.1) but I can't see that written down anywhere. I'd like to be able to avoid any of the YAML 1.1 issues (such as the infamous no boolean).

Furthermore, I can't see any mention of selecting a schema when parsing or generating. Does serde-yaml implement the Core, the JSON or the Failsafe schema?