Mention SQLite database format change in UPGRADING file (#1488983)

CHANGELOG

@@ -1,6 +1,7 @@
 CHANGELOG Roundcube Webmail
 ===========================
 
+- Mention SQLite database format change in UPGRADING file (#1488983)
 - Increase maxlength to 254 chars for email input fields in addressbook (#1488987)
 - Fix thumbnail size when GD extension is used for image resize (#1488985)
 - Display notice that message is encrypted also for application/pkcs7-mime messages (#1488526)

UPGRADING

@@ -40,6 +40,7 @@ it on a unix system, you need to do the following operations by hand:
    open http://url-to-roundcube/installer/ in a browser and choose "3 Test config".
    To enable the latter one, you have to temporary set 'enable_installer'
    to true in your local config/main.inc.php file.
+   WARNING: See SQLite database upgrade below.
 4. Let the update script/installer check your configuration and
    update your config files and database schema as suggested by the updater.
 5. Make sure 'enable_installer' is set to false again.
@@ -54,3 +55,11 @@ Post-Upgrade Activities
    your folder settings contain namespace prefix. For example Courier users
    should add INBOX. prefix to folder names in main configuration file.
 4. Check system requirements in INSTALL file.
+
+SQLite database upgrade
+-----------------------
+Versions older than 0.9 were supporting SQLite v2 only. Newer versions require
+database in v3 format. The best what you can do is to convert database file
+to the new format using command line tools:
+
+sqlite OLD.DB .dump | sqlite3 NEW.DB