docs: add mocking info to unit testing guide
Resolves
@@ -203,6 +203,19 @@ you will find the per-function analysis useful. This will show you code paths th
![per function](./images/unit-test-coverage-3.png)
+##### Mocking
+
+OSM uses the [GoMock](https://github.com/golang/mock) mocking framework to mock interfaces in unit tests.
+GoMock's `mockgen` tool is used to autogenerate mocks from interfaces.
+
+As an example, to create a mock client for the `Configurator` interface defined in the [configurator](/pkg/configurator) package:
+```bash
+mockgen -destination=pkg/configurator/mock_client.go -package=configurator github.com/openservicemesh/osm/pkg/configurator Configurator
+```
+
+When a mocked interface is changed, the autogenerated mock code must be regenerated.
+More details can be found in [GoMock's documentation](https://github.com/golang/mock/blob/master/README.md).
+
#### Integration Tests
Unit tests focus on a single function. These ensure that with a specific input, the function
