Add note to README about `migrate` compatibility.
Add note to README about `migrate` compatibility.

Once ckan/ckan@796e95412c9edc05f44181a53ad86c3147eb24ac is released
(likely CKAN 2.6), the showcase `migrate` command will no longer work.
This commit adds a note to the README advising on upgrade strategy.

ckan/ckanext-showcase,ckan/ckanext-showcase,ckan/ckanext-showcase