Refs #13948: Add warning note stating that STATISTICS must be enabled…

… for the Backend to collect data

Signed-off-by: JLBuenoLopez-eProsima <joseluisbueno@eprosima.com>

docs/rst/formalia/titlepage.rst

@@ -21,3 +21,11 @@ generate statistical information to be used by applications.
  This library is in the early stages of development.
  For this reason, the API should not be considered stable, as API breaks may occur before the first official release.
  Furthermore, several features may not be implemented yet.
+
+.. warning::
+ Be aware that the statistics module is not compiled and used by default by *Fast DDS* but it has to be specifically
+ configured to send statistical data of an specific entity.
+ Therefore, *eProsima Fast DDS* library must be compiled with `FASTDDS_STATISTICS` CMake option enabled and use either
+ the `FASTDDS_STATISTICS` environment variable or the DomainParticipantQos to activate the statistics DataWriters.
+ For more information, please refer to the
+ `Statistics Module documentation <https://fast-dds.docs.eprosima.com/en/latest/fastdds/statistics/statistics.html#statistics-module>`_.