README: add badge for build status
README: add badge for build status

frobware/cmd-key-happy,andymcd/cmd-key-happy,frobware/cmd-key-happy,andymcd/cmd-key-happy,andymcd/cmd-key-happy,andymcd/cmd-key-happy,frobware/cmd-key-happy,frobware/cmd-key-happy,frobware/cmd-key-happy,andymcd/cmd-key-happy