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.