Fixed #15940 -- Added MySQL note about strict SQL mode

docs/ref/databases.txt

@@ -439,6 +439,24 @@ more details.
 .. _MySQL option file: https://dev.mysql.com/doc/refman/5.6/en/option-files.html
 .. _MySQLdb documentation: http://mysql-python.sourceforge.net/
 
+.. _mysql-sql-mode:
+
+Setting ``sql_mode``
+~~~~~~~~~~~~~~~~~~~~
+
+From MySQL 5.7 onwards and on fresh installs of MySQL 5.6, the default value of
+the ``sql_mode`` option contains ``STRICT_TRANS_TABLES``. That option escalates
+warnings into errors when data are truncated upon insertion, so Django highly
+recommends activating a `strict mode`_ for MySQL to prevent data loss (either
+``STRICT_TRANS_TABLES`` or ``STRICT_ALL_TABLES``).
+
+.. _strict mode: https://dev.mysql.com/doc/refman/5.7/en/sql-mode.html#sql-mode-strict
+
+If you need to customize the SQL mode, you can set the ``sql_mode`` variable
+like other MySQL options: either in a config file or with the entry
+``'init_command': "SET sql_mode='STRICT_TRANS_TABLES'"`` in the
+:setting:`OPTIONS` part of your database configuration in :setting:`DATABASES`.
+
 Creating your tables
 --------------------