Document how to use a Diff class to control how the diff between 2 adapters is done

[dgarros]

Environment

• DiffSync version: 1.3.0

Proposed Functionality

Update the documentation to explain how to create and use a custom Diff class

For reference,

• The first example is using a diff class
• The Network-Importer is using a custom Diff class here

[dgarros]

The doc has been updated