Update READ.me to make part of mockery more clear

[timnew]

Kind of related to issue #70

• Register mock node-fetch instead of fetch, since node-fetch is used in the code, which is more straightforward.
• Move deregisterMock call to afterEach hook, since current implementation might cause mock leak if test code failed
• Add the instruction to use require rather than import

[wheresrhys]

I've just merged some biggish refactors and changes to documentation, including a lot more on use with mockery. I think all your points are addressed, but feel free to open another PR if you can think of any improvements

[timnew]

OK

[timnew]

I still think current implementation might cause mock leak when test failed.

Since when assertion failed, the mockery.deregisterMock('node-fetch') will never get executed.
An easy and safe way to solve the issue is to move this call into afterEach hook, which is guaranteed to be executed after every test, whether it is passed or not.

And there is a typo: mockery.deregisterMock('fetch') should be mockery.deregisterMock('node-fetch')