v: add a warning note to `v repl`

cmd/v/v.v

@@ -61,6 +61,8 @@ fn main() {
 		if args.len == 0 {
 			if os.is_atty(0) != 0 {
 				println('Welcome to the V REPL (for help with V itself, type `exit`, then run `v help`).')
+				eprintln('  NB: the REPL is highly experimental. For best V experience, use a text editor,')
+				eprintln('  save your code in a `main.v` file and do: `v run main.v`')
 			} else {
 				mut args_and_flags := util.join_env_vflags_and_os_args()[1..].clone()
 				args_and_flags << ['run', '-']