Add mandoc-manpage for rm(1)

and mark it as finished in README.

rm.c

@@ -10,7 +10,7 @@
 static void
 usage(void)
 {
-	eprintf("usage: %s [-fRr] FILE...\n", argv0);
+	eprintf("usage: %s [-f | -i] [-Rr] file ...\n", argv0);
 }
 
 int
@@ -20,6 +20,9 @@ main(int argc, char *argv[])
 	case 'f':
 		rm_fflag = 1;
 		break;
+	case 'i':
+		rm_fflag = 0;
+		break;
 	case 'R':
 	case 'r':
 		rm_rflag = 1;