README: Add instructions for how to mock/test code that use Algolia

[runephilosof-abtion]

In v1 of this gem there was a webmock configuration that you could just require.
That was removed in v2 (c88b821a951d9b685e47dd6ca369f352b2a3e7dd)

I have done as suggested in algolia/algoliasearch-rails#398 (comment) and copied the MockRequester from this repos testing suite.

It would be nice if the README contained hints for how to test code using it.