Adds example for csv_headers

Author: lajosthiel

components/serializer.rst

@@ -897,7 +897,14 @@ Option                  Description                                           De
 ``csv_escape_char``     Sets the escape character (at most one character)
 ``csv_key_separator``   Sets the separator for array's keys during its        ``.``
                         flattening
-``csv_headers``         Sets the order of the header and data columns         ``[]``, inferred from input data's keys
+``csv_headers``         Sets the order of the header and data columns
+                        E.g.: $this->serializer->serialize(
+                            ['c' => 3,'a' => 1,'b' => 2],
+                            CsvEncoder::FORMAT,
+                            [CsvEncoder::HEADERS_KEY => ['a', 'b', 'c']]);
+                        returns
+                            a,b,c
+                            1,2,3                                             ``[]``, inferred from input data's keys
 ``csv_escape_formulas`` Escapes fields containg formulas by prepending them   ``false``
                         with a ``\t`` character
 ``as_collection``       Always returns results as a collection, even if only