Add readme to openmrs/modules/dtd
to explain what they do, where to find further information and how they
are synced to http://resources.openmrs.org/doctype
TRUNK-5404 Deprecate unused MigrationHelper
since it is not used within core and not anywhere in the openmrs github
organization and has only been updated a few times to keep it from
breaking our build and up to our coding standards.
