Add zsh completion instructions (fix #1420)
Add zsh completion instructions (fix #1420)

madmouser1/beets,Andypsamp/CODjunit,m-urban/beets,sampsyo/beets,Dishwishy/beets,xsteadfastx/beets,Andypsamp/CODfinalJUNIT,ttsda/beets,kelvinhammond/beets,shanemikel/beets,untitaker/beets,asteven/beets,tima/beets,dfc/beets,jmwatte/beets,pkess/beets,Andypsamp/CODjunit,jackwilsdon/beets,parapente/beets,Andypsamp/CODfinalJUNIT,beetbox/beets,madmouser1/beets,lengtche/beets,arabenjamin/beets,pkess/beets,sadatay/beets,kareemallen/beets,jackwilsdon/beets,jcoady9/beets,Dishwishy/beets,Dishwishy/beets,parapente/beets,sampsyo/beets,mried/beets,swt30/beets,mosesfistos1/beetbox,LordSputnik/beets,Andypsamp/CODjunit,mathstuf/beets,asteven/beets,moodboom/beets,mosesfistos1/beetbox,PierreRust/beets,lightwang1/beets,lightwang1/beets,swt30/beets,mosesfistos1/beetbox,LordSputnik/beets,kelvinhammond/beets,MyTunesFreeMusic/privacy-policy,pkess/beets,SusannaMaria/beets,jackwilsdon/beets,lightwang1/beets,moodboom/beets,untitaker/beets,sadatay/beets,Andypsamp/CODfinalJUNIT,PierreRust/beets,imsparsh/beets,gabrielaraujof/beets,ttsda/beets,shamangeorge/beets,diego-plan9/beets,marcuskrahl/beets,ruippeixotog/beets,mosesfistos1/beetbox,moodboom/beets,lightwang1/beets,m-urban/beets,pkess/beets,mried/beets,tima/beets,Freso/beets,kareemallen/beets,diego-plan9/beets,parapente/beets,ibmibmibm/beets,ruippeixotog/beets,jmwatte/beets,dfc/beets,artemutin/beets,imsparsh/beets,MyTunesFreeMusic/privacy-policy,YetAnotherNerd/beets,moodboom/beets,artemutin/beets,lengtche/beets,YetAnotherNerd/beets,marcuskrahl/beets,lengtche/beets,jackwilsdon/beets,beetbox/beets,YetAnotherNerd/beets,shamangeorge/beets,lengtche/beets,dfc/beets,xsteadfastx/beets,MyTunesFreeMusic/privacy-policy,ibmibmibm/beets,parapente/beets,shanemikel/beets,xsteadfastx/beets,shamangeorge/beets,ruippeixotog/beets,tima/beets,beetbox/beets,YetAnotherNerd/beets,asteven/beets,SusannaMaria/beets,jcoady9/beets,Dishwishy/beets,SusannaMaria/beets,LordSputnik/beets,untitaker/beets,drm00/beets,diego-plan9/beets,shamangeorge/beets,ttsda/beets,untitaker/beets,kareemallen/beets,marcuskrahl/beets,Kraymer/beets,swt30/beets,asteven/beets,kelvinhammond/beets,imsparsh/beets,shanemikel/beets,jcoady9/beets,kareemallen/beets,xsteadfastx/beets,kelvinhammond/beets,sadatay/beets,diego-plan9/beets,gabrielaraujof/beets,artemutin/beets,drm00/beets,sadatay/beets,Kraymer/beets,ruippeixotog/beets,randybias/beets,beetbox/beets,mried/beets,randybias/beets,Freso/beets,jmwatte/beets,gabrielaraujof/beets,ibmibmibm/beets,tima/beets,madmouser1/beets,artemutin/beets,marcuskrahl/beets,Freso/beets,ttsda/beets,dfc/beets,madmouser1/beets,randybias/beets,arabenjamin/beets,shanemikel/beets,mathstuf/beets,SusannaMaria/beets,arabenjamin/beets,Andypsamp/CODfinalJUNIT,drm00/beets,randybias/beets,LordSputnik/beets,PierreRust/beets,mathstuf/beets,Andypsamp/CODjunit,swt30/beets,m-urban/beets,Andypsamp/CODfinalJUNIT,Freso/beets,sampsyo/beets,MyTunesFreeMusic/privacy-policy,drm00/beets,mried/beets,Kraymer/beets,sampsyo/beets,jcoady9/beets,m-urban/beets,imsparsh/beets,ibmibmibm/beets,Andypsamp/CODjunit,PierreRust/beets,mathstuf/beets,jmwatte/beets,gabrielaraujof/beets,Kraymer/beets,arabenjamin/beets