docs: README.md note login to download auto-builds

Add note to README.md explaining that one must log into GitHub to
download artifacts, i.e. the automated Windows and macOS builds.  This
does not impact full releases.

This commit can be reverted once the GitHub issue is addressed.

See actions/upload-artifact#51

README.md

@@ -22,6 +22,8 @@ Official, stable downloads are provided on the [Quassel IRC download page](https
 
 Automated Windows and macOS builds are available via the [GitHub Actions tab][ci-status-page].  Pick a build, then download the appropriate artifact.
 
+*NOTE: As of this writing, you need to log into GitHub to download the automated builds.  This [issue is being tracked with GitHub](https://github.com/actions/upload-artifact/issues/51 ).*
+
 Unofficial builds and testing versions are [contributed by several community members](https://bugs.quassel-irc.org/projects/quassel-irc/wiki#Unofficial-builds).
 
 ## Quick reference