Add --timeout notice (#557)

Closes: https://github.com/shssoichiro/oxipng/issues/556

Adds a statement saying that `--timeout` isn't as useful for compression
algorithms which use fewer and slower rounds, which OxiPNG tends to use
nowadays.

---------

Co-authored-by: andrews05 <andrew@digerati.co.nz>

src/main.rs

@@ -257,7 +257,7 @@ fn main() {
         )
         .arg(
             Arg::new("timeout")
-                .help("Maximum amount of time, in seconds, to spend on optimizations")
+                .help("Maximum amount of time, in seconds, to spend on optimizations (currently of limited use due to the shift away from zlib)")
                 .value_name("secs")
                 .long("timeout")
                 .value_parser(value_parser!(u64)),