### Environment * DiffSync version: 1.3.0 ### Proposed Functionality Update the documentation to explain how to create and use a custom [Diff class](https://github.com/networktocode/diffsync/blob/master/diffsync/diff.py#L25) For reference, - The [first example](https://github.com/networktocode/diffsync/blob/master/examples/example1/main.py) is using a diff class - The Network-Importer is using a custom Diff class [here](https://github.com/networktocode/network-importer/blob/develop/network_importer/diff.py#L19)