add comment for emitUTF8 in header

Author: PinkD

include/json/writer.h

@@ -110,6 +110,8 @@ class JSON_API StreamWriterBuilder : public StreamWriter::Factory {
    *  - Number of precision digits for formatting of real values.
    *  - "precisionType": "significant"(default) or "decimal"
    *  - Type of precision for formatting of real values.
+   *  - "emitUTF8": false or true
+   *  - If true, outputs raw UTF8 strings instead of escaping them.
 
    *  You can examine 'settings_` yourself
    *  to see the defaults. You can also write and read them just like any