Update the README to reflect the current situation

[fwcd]

Many of the blockers mentioned in the README (in particular waveforms requiring a legacy widget to render, thus preventing an update to a newer Qt version) are no longer applicable as of

• qopenglwindow-based replacement of qglwidget, full glsl shader based waveforms, vumeters and spinnies mixxxdj/mixxx#10989
• Build with Qt6 and optionally with QML mixxxdj/mixxx#11608

We should therefore update the description to reflect the current status and ideally make the link to the official ARM snapshots more prominent (especially once a stable ARM version is released).

There are also still a few minor issues with the official snapshots that this unofficial build doesn't have, in particular this signing/sandbox-related issue:

• macOS sandboxing: User reprompted for folder permission at every launch mixxxdj/mixxx#11552