Add mandoc-manpage for mv(1)

and mark it as finished in README.
Make it clear that -i has not been implemented yet and drop the
flag in the code instead of erroring out.

mv.c

@@ -13,7 +13,7 @@ static int mv(const char *, const char *);
 static void
 usage(void)
 {
-	eprintf("usage: %s [-f] source... dest\n", argv0);
+	eprintf("usage: %s [-f | -i] source... dest\n", argv0);
 }
 
 int
@@ -22,6 +22,7 @@ main(int argc, char *argv[])
 	struct stat st;
 
 	ARGBEGIN {
+	case 'i':
 	case 'f':
 		break;
 	default: