docs: Add nightly.link CI download URLs to README

Add nightly.link CI download URLs for Windows and macOS to the README
file.  This provides a convenient way for anonymous access to the
latest builds, getting around the requirement of being signed in.

NOTE: To avoid future rate-limiting by GitHub, nightly.link should be
given an access token (installed as a GitHub app) so artifact URL
fetches won't share the default API token rate limits.

See https://nightly.link/ and use the option to install and select
one's repositories.  Or go directly to...
https://github.com/apps/nightly-link

If you wish to provide direct download links to your fork of Quassel,
you may want to do the same.