UCM2 documentation

[GitHub issues - opened]

alsa-project/alsa-ucm-conf issue #103 was opened from perexg:

Add/improve the documentation for

- directory tree layout
- build-in string substitutions
- runtime conditional configuration selection
- mixer settings (boot -> use -> verb -> device)

Issue URL     : https://github.com/alsa-project/alsa-ucm-conf/issues/103
Repository URL: https://github.com/alsa-project/alsa-ucm-conf