docs: Add upmpdcli as an alternative to Rygel

Based on PR #871 by @woutervanwijk.

Fixes #871
Add appendinx about xbmc issues

XBMC won't boot because a different libtag version got
installed with mopidy system wide, and XBMC picks that up
insted of the one it vendors in. Added info how to fix this.
docs: Edit Raspbmc appendix a bit
Merge branch 'master' into release/v0.19.x