Update documentation on upgrading MapIt
Update documentation on upgrading MapIt

- add command for getting checksum
- note that Puppet won't run cleanly while changes to update the database have
  been deployed but not yet run
- recommend using fabric script to run the upgrade; I'm moving the comments
  from the commands into that script so that we know why each step is
  necessary

alphagov/govuk-puppet,alphagov/govuk-puppet,alphagov/govuk-puppet,alphagov/govuk-puppet,alphagov/govuk-puppet,alphagov/govuk-puppet