'zrepl version' and 'zrepl control version' subcommand + maintainer README

Version is autodetected on build using git
If it cannot be detected with git, an override must be provided.

For tracability of distros, the distroy packagers should override as
well, which is why I added a README entry for package mainatiners.

refs #35

cmd/main.go

@@ -20,6 +20,10 @@ import (
 //	Printf(format string, v ...interface{})
 //}
 
+var (
+	zreplVersion string // set by build infrastructure
+)
+
 type Logger = *logger.Logger
 
 var RootCmd = &cobra.Command{