minor symfony#11409 Add a note about a possible SQLite error (javiereguiluz)

This PR was squashed before being merged into the 3.4 branch (closes symfony#11409).

Discussion
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Add a note about a possible SQLite error

Fixes symfony#9424.

Commits
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9d09fce Add a note about a possible SQLite error

Author: javiereguiluz

doctrine.rst

@@ -391,6 +391,13 @@ directly inside the ``Product`` class via DocBlock annotations:
             </entity>
         </doctrine-mapping>
 
+.. note::
+
+    If you are using an SQLite database, you'll see the following error:
+    *PDOException: SQLSTATE[HY000]: General error: 1 Cannot add a NOT NULL
+    column with default value NULL*. Add a ``nullable=true`` option to the
+    ``description`` property to fix the problem.
+
 .. note::
 
     A bundle can accept only one metadata definition format. For example, it's