Update readome for migration.
Update readome for migration.

For auto-migration case, newlly added column must have defultExpr
or @Nullable explicitly, but this was only explained Japanese blog entry.
http://gfx.hatenablog.com/entry/2016/01/12/093529

gfx/Android-Orma,gfx/Android-Orma,gfx/Android-Orma,gfx/Android-Orma