Add 'Upgrading WCT database schema' section.
Add 'Upgrading WCT database schema' section.

- Add a section covering generic WCT schema upgrades. Specific notes for a
  particular release's database changes should be included in the
  'release-notes.rst'

DIA-NZ/webcurator,DIA-NZ/webcurator,DIA-NZ/webcurator,DIA-NZ/webcurator