Update 'src/main.rs'

src/main.rs

@@ -139,6 +139,7 @@ fn to_readable_units(n: usize, how_units: HowUnits, how_base: HowNumberBase) ->
 }
 
 const USAGE: &str = "Usage: cntin [options]
+Counts bytes in stdin.
 Note: If both of a complementary set of options are specified, both are used.
    options:
       -d, --decimal    Sets the output number base to decimal.