Documentation should mention how to handle default-export imports

[jtarvainen]

The documentation should mention how to handle default-export imports. For example, in the following case:

1. We want to test the default export main from main.ts,
2. main.ts imports the default export importedFunction from importedFunction.ts, and
3. We want to stub importedFunction with importedFunctionStub;

then we need to do this:

const { default: main } = proxyquire<{ default: typeof main }>(
    'path/to/main', {
        'path/to/importedFunction': { default: importedFunctionStub },
    }
);

[bendrucker]

Please contribute the documentation you think you help.