Adding note about the PdoSessionHandler BC break

UPGRADE-2.6.md

@@ -106,6 +106,20 @@ HttpFoundation
 --------------
 
  * The `PdoSessionHandler` to store sessions in a database changed significantly.
+   This introduced a **backwards-compatability** break in the schema of the
+   session table. The following changes must be made to your session table:
+
+   - Add a new integer column called `sess_lifetime`. Assuming you have the
+     default column and table names, in MySQL this would be:
+       ALTER TABLE `session` ADD `sess_lifetime` INT NOT NULL ;
+   - Change the data column (default: `sess_value`) to be a Blob type. In
+     MySQL this would be:
+      ALTER TABLE `session` CHANGE `sess_value` `session_value` BLOB NOT NULL;
+
+   There is also an [issue](https://github.com/symfony/symfony/issues/12834)
+   that affects Windows servers.
+
+   The changes to the `PdoSessionHandler` are:
    - By default, it now implements session locking to prevent loss of data by concurrent access to the same session.
      - It does so using a transaction between opening and closing a session. For this reason, it's not
        recommended to use the same database connection that you also use for your application logic.