Include c0 in README

Author: iryont

README.md

@@ -23,7 +23,7 @@ You can use < or > at the beginning of the format string to specify the byte ord
 "f" a float (4 bytes).
 "d" a double (8 bytes).
 "s" a zero-terminated string.
-"cn" a sequence of exactly n chars corresponding to a single Lua string.
+"cn" a sequence of exactly n chars corresponding to a single Lua string (if n <= 0 then the string length is taken).
 ```
 ### how to use it?
 ```lua