Document how to mock prismjs in consumers

[kylesuss]

Mocking prismjs was a bit difficult in one of the consumers. I tried this in the test file:

jest.mock('prismjs/components/index');
jest.mock('prismjs', () => ({
  highlightAllUnder: jest.fn(),
}));

Sadly that did not work & I had to create mock files like this:

// __mocks__/prismjs/components/index.js
const loadLanguages = jest.fn();

export default loadLanguages;

// __mocks__/prismjs.js 
export default {
  highlightAllUnder: jest.fn(),
};

Figure out if that is the best way to do this & add something to the README about it so others don't waste time on it.