Document skipBom in CharReaderBuilder (#1332)

Author: axic

include/json/reader.h

@@ -324,6 +324,9 @@ class JSON_API CharReaderBuilder : public CharReader::Factory {
    * - `"allowSpecialFloats": false or true`
    *   - If true, special float values (NaNs and infinities) are allowed and
    *     their values are lossfree restorable.
+   * - `"skipBom": false or true`
+   *   - If true, if the input starts with the Unicode byte order mark (BOM),
+   *     it is skipped.
    *
    * You can examine 'settings_` yourself to see the defaults. You can also
    * write and read them just like any JSON Value.