Update docs to describe difference between pyyaml and ryaml

[jonathan-s]

There are some crucial differences between pyyaml and ryaml which should be described as they implement different versions of the yaml spec.

The differences is also described in another python package that implements the 1.2 version of the spec. https://yaml.readthedocs.io/en/latest/pyyaml.html#defaulting-to-yaml-1-2-support

closes #3