Adds a recommended config with new no-assigning-return-values rule

[SaladFork]

Adds an opinionated rule as suggested in #4. Tries to help with the "Assigning Return Values" Best Practice described in the guide, something that bit me when I was first trying out Cypress.

When writing the rules and the folder layout, I tried to follow the practices described in the ESLint guide. I also chose jest as the test runner but it can be easily switched to Mocha or whatever a maintainer would prefer.

I also updated the readme to instruct on how to add the recommended config, as well as a list of rules and instructions for developing new rules.

Known shortcomings:

• This rule should catch any assignment type (let, const, var). However, it will not catch more complex assignment operations such as destructuring: let [a, b] = [true, cypress.get('foo')].

Feedback/ideas on this or other potential rules welcome.

---

Example:

// cypress/.eslintrc.js
module.exports = {
  plugins: ['cypress'],
  extends: 'plugin:cypress/recommended',
  env: {
    'cypress/globals': true
  }
}

// cypress/test.js
describe('something', () => {
  it('might do a thing', () => {
    let a = cy.contains('Submit');
    a.click();
  });
});

$ yarn run eslint cypress/test.js

./cypress/test.js
  3:5  error  Do not assign the return value of a Cypress command  cypress/no-assigning-return-values

✖ 1 problem (1 error, 0 warnings)

[chrisbreiding]

Very nice! Thanks for contributing this. I added just one minor change request, but otherwise it looks good. Once this is in, I'll add a CircleCI config so the tests get run in CI.

[chrisbreiding]

Released in v2.1.0 🎉