Update cli usage in README

README.md

@@ -78,8 +78,9 @@ Output of `mincss -h`:
     Generic options:
     -h, --help        Show this help message
     -v, --verbose     Verbose mode: show compression rate
-     -o <file>         Output file (defaults to stdout)
-     <file> ...        Input files (default is to read from stdin)
+    -o <file>
+    --output=<file>   Output file (defaults to stdout)
+    <file> ...        Input files (defaults to stdin or "-")
 
     Optimization flags (default is -w -c -s -d):
     -w, --whitespace  Eliminate unnecessary whitespaces (has the greatest effect, omit for pretty-printing)