better docs on handling migrations..

ui/medashare/alembic/README

@@ -1,7 +1,36 @@
 New Version
 -----------
 
-To create a revision, update the ORM in medashare/orm.py.
+First make a test in `_TestMigrations` with the data that you will need.
+Use the first 5 characters for the test name.  Then dump the data to a
+file so that it can be read back in.  From the test fixture that setups
+the data, use the code:
+```
+tf = pathlib.Path('fixtures/test_<oldversion>.sqlite').absolute()
+engine_file = sqlalchemy.create_engine('sqlite+pysqlite:///' + str(tf))
+raw_connection_file = engine_file.raw_connection()
+
+objcon.backup(raw_connection_file.connection)
+
+raw_connection_file.close()
+engine_file.dispose()
+objcon.close()
+```
+
+Then you can comment it out, or use `if False: #pragma: no cover` and
+replace it with the following to load the saved database:
+```
+<tf from above>
+objcon = self._engine.raw_connection()
+engine_file = sqlalchemy.create_engine('sqlite+pysqlite:///' + str(tf))
+raw_connection_file = engine_file.raw_connection()
+raw_connection_file.connection.backup(objcon.connection)
+raw_connection_file.close()
+engine_file.dispose()
+objcon.close()
+```
+
+Update the ORM in `medashare/orm.py` as needed.
 
 Then in the medashare directory, run:
 ```
@@ -9,7 +38,10 @@ Then in the medashare directory, run:
 ```
 
 This will create a version file.  Edit the version file to support the
-migration (such as populating new columns).
+migration (such as populating new columns or tables).
+
+Finish the test for the migration.  Note, you may need to update code
+to handle older versions.  This can be handled by calling xxx. (TODO)
 
 Once things are tested as good, commit everything.