docs: Improve readme
docs: Improve readme

Use text from www.mopidy.com. Change "Download development snapshot" link,
which confused some new users.

bacontext/mopidy,woutervanwijk/mopidy,glogiotatidis/mopidy,diandiankan/mopidy,SuperStarPL/mopidy,ali/mopidy,glogiotatidis/mopidy,swak/mopidy,adamcik/mopidy,swak/mopidy,mopidy/mopidy,bencevans/mopidy,dbrgn/mopidy,priestd09/mopidy,ZenithDK/mopidy,dbrgn/mopidy,priestd09/mopidy,bencevans/mopidy,liamw9534/mopidy,jmarsik/mopidy,diandiankan/mopidy,mopidy/mopidy,hkariti/mopidy,ali/mopidy,tkem/mopidy,rawdlite/mopidy,jodal/mopidy,jcass77/mopidy,dbrgn/mopidy,rawdlite/mopidy,pacificIT/mopidy,pacificIT/mopidy,mokieyue/mopidy,mokieyue/mopidy,jodal/mopidy,adamcik/mopidy,vrs01/mopidy,woutervanwijk/mopidy,mopidy/mopidy,bacontext/mopidy,quartz55/mopidy,bacontext/mopidy,kingosticks/mopidy,bencevans/mopidy,glogiotatidis/mopidy,mokieyue/mopidy,ali/mopidy,jcass77/mopidy,adamcik/mopidy,jmarsik/mopidy,mokieyue/mopidy,hkariti/mopidy,quartz55/mopidy,quartz55/mopidy,tkem/mopidy,hkariti/mopidy,rawdlite/mopidy,ZenithDK/mopidy,diandiankan/mopidy,kingosticks/mopidy,jcass77/mopidy,swak/mopidy,pacificIT/mopidy,jmarsik/mopidy,pacificIT/mopidy,SuperStarPL/mopidy,SuperStarPL/mopidy,tkem/mopidy,rawdlite/mopidy,glogiotatidis/mopidy,ZenithDK/mopidy,ZenithDK/mopidy,priestd09/mopidy,vrs01/mopidy,kingosticks/mopidy,jodal/mopidy,bacontext/mopidy,SuperStarPL/mopidy,vrs01/mopidy,swak/mopidy,bencevans/mopidy,jmarsik/mopidy,hkariti/mopidy,dbrgn/mopidy,tkem/mopidy,ali/mopidy,vrs01/mopidy,liamw9534/mopidy,quartz55/mopidy,diandiankan/mopidy