README.md: change instructions to use release builds.
README.md: change instructions to use release builds.

On embedded devices (which many people use librespot on), debug builds
are too slow and produce choppy audio.
XDjackieXD/librespot,bobbo/spotify-connect-scrobbler,bobbo/spotify-connect-scrobbler,SimonPersson/librespot,joerg-krause/librespot,XDjackieXD/librespot,joerg-krause/librespot,michaelherger/librespot,plietar/spotify-connect,plietar/librespot,michaelherger/librespot,plietar/librespot