Document the use_nil option for encoding JSON

README.md

@@ -71,6 +71,7 @@ The options for encode are:
 * `force_utf8` - Force strings to encode as UTF-8 by fixing broken
   surrogate pairs and/or using the replacement character to remove
   broken UTF-8 sequences in data.
+* `use_nil` - Encode's the atom `nil` as `null`.
 * `escape_forward_slashes` - Escapes the `/` character which can be
   useful when encoding URLs in some cases.
 * `{bytes_per_red, N}` - Refer to the decode options