readme: Add a Cirrus build badge
readme: Add a Cirrus build badge

jkonecny12/kakoune,danr/kakoune,occivink/kakoune,lenormf/kakoune,lenormf/kakoune,alexherbo2/kakoune,alexherbo2/kakoune,jkonecny12/kakoune,danr/kakoune,mawww/kakoune,lenormf/kakoune,occivink/kakoune,Somasis/kakoune,Somasis/kakoune,casimir/kakoune,alexherbo2/kakoune,Somasis/kakoune,jkonecny12/kakoune,jkonecny12/kakoune,alexherbo2/kakoune,mawww/kakoune,danr/kakoune,lenormf/kakoune,occivink/kakoune,casimir/kakoune,mawww/kakoune,Somasis/kakoune,occivink/kakoune,mawww/kakoune,casimir/kakoune,casimir/kakoune,danr/kakoune