Docs: add simpler solution to implement (closes #181)

[JPeer264]

I did not include automock: false as this is false by default.

(ref: #181)

[yinzara]

This an interesting proposal. I was not aware jest supported this kind of syntax. There is actually additional code changes required so I'll take this myself.

[JPeer264]

@yinzara the only thing which is missing is a self execution, I guess the fasted would be to enable mocks before exporting the mock.

+ fetch.enableMocks();

module.exports = fetch.default = fetch

Sorry for assuming this already works, I was too much in rush back then.