Clarify parent/chained tokens support

[jbasney]

I think d30cb10 removed the parent/chained tokens support, but README.rst says

NOTE: SciTokens (the format and this library) is currently being designed; this README describes how we would like it to work, not necessarily current functionality. Particularly, we do not foresee the chained tokens described here as part of the first release's functionality.

and

SciTokens can be chained, meaning one token can be appended to another:
token = scitokens.SciToken(parent=parent_token)
The generated object, token, will default to having all the authoriations of the parent token - but is mutable and can add further restrictions.

and there's still a parent parameter

scitokens/src/scitokens/scitokens.py

Line 39 in 06d8d10

:param parent: Parent SciToken that will be chained

Since there have been 10+ releases, I'd like to update that NOTE in the README to be clearer about the stability of the format/library and remove the references to parent/chained tokens unless that's still supported/planned.

I can make a pull request, but I wanted to check first.

[djw8605]

Oddly, just saw this. Yes, feel free to make a pull request. The parent changed tokens is not supported, never was supported, and is not planned to be supported.