Merge branch 'make' into 'master'

add makefile to clarify how this is installed

See merge request !2

Makefile

+PREFIX?=/usr
+
+install:
+	cp -p src/once.sh ${DESTDIR}${PREFIX}/bin/once
+	cp -p src/completion/once.bash-completion ${DESTDIR}${PREFIX}/share/bash-completion/completions/once