olddoc 1.1.0 - NNTP and more

This release adds support for the `nntp_url` in .olddoc.yml
for mailing lists with NNTP gateways (possibly read-only).

This exists because the project mailing list is now readable
over NNTP:

  nntp://news.public-inbox.org/inbox.comp.lang.ruby.olddoc

This also relaxes the dependency on the README being
RDoc-formatted, allowing "olddoc prepare" to generate
NEWS/NEWS.atom.xml files for projects which use only
plain-text and not RDoc.

* support for nntp_url, make private_email optional
* gemspec: use SPDX-compatible license
* readme: relax dependency on RDoc formatting