add man entry for --max

programs/zstd.1.md

@@ -161,6 +161,9 @@ the last one takes effect.
 
     Note: If `windowLog` is set to larger than 27, `--long=windowLog` or
     `--memory=windowSize` needs to be passed to the decompressor.
+* `--max`:
+    set advanced parameters to maximum compression.
+    warning: this setting uses a lot of resources and is very slow.
 * `-D DICT`:
     use `DICT` as Dictionary to compress or decompress FILE(s)
 * `--patch-from FILE`: