Add Import/Export development `lib/gitlab/import_export/README.md`
Document best development practices regarding import / export feature.
It is not clear when to bump internal version of import/export schema. It is not clear how import / export works. It often happens that RSpec test fails, and suggests adding an entry to
spec/lib/gitlab/import_export/safe_model_attributes.yml but this is not a good solution when there is a breaking change (renaming attribute / column). It is not documented what is a breaking change.
We might want to add
lib/gitlab/import_export/README.md with some details about best development practices.