Add documention clarifying use of scenarios (#1427)

[allthesignals]

This PR adds some documentation about scenarios and ways to use them more effectively.

[samselikoff]

Requested changes for first three paragraphs: generalize the motivation a bit (not just user acceptance testing), and open with the bit about not being a first-class API. Something like:

Mirage adds a /scenarios directory to your project, where you'll find a default.js file. The function in this file will run in development mode and act as your "development seeds". In testing it's ignored, so that your Mirage server starts out with a clean database. You can then use each test to seed Mirage as needed for that test. In this way, each test acts as its own Mirage scenario.

Originally Mirage was going to add first-class support for development-time scenario switching, but right only the the scenarios/default.js file is used by Mirage. However, you can use various strategies to configure the behavior of your default scenario, to support multiple development server states or to facilitate things like running user acceptance testing. For example, you might want to test whether a user can complete a set of tasks specific to unique circumstances defined in a scenario.

One strategy you could take is to configure your default scenario file to honor options passed to the runtime environment. For example...

[samselikoff]

Let's move this part first, so as we're explaining the reader knows the starting point. Let's also avoid the private server._config.

---

One strategy would be to use ENV vars to switch scenarios. For example, say you wanted to be able to run a scenario file like this:

MIRAGE_SCENARIO=administrator ember s

To use this MIRAGE_SCENARIO ENV variable to alter the behavior of scenarios/default.js, add it to your environment/config.js:

const { MIRAGE_SCENARIO } = process.env;
module.exports = function(environment) {
  const ENV = {
    // ...other stuff

    MIRAGE_SCENARIO,

    // ...other stuff
  };

  return ENV;
}

then access this new config variable in your scenarios/default.js file:

// scenarios/default.js
import ENV from '../../config/environment';
import visitor from './visitor';
import subscriber from './subscriber';
import administrator from './administrator';

const scenarios = {
  visitor,
  subscriber,
  administrator
}

const activeScenario = ENV.MIRAGE_SCENARIO || 'visitor';

export default function(server) {
  const scenario = scenarios[activeScenario];

  if (scenario) {
    scenario(server);
  }

  // plus whatever default scenario code you want
}

[samselikoff]

Nice work, added some comments!

[allthesignals]

@samselikoff thank you for the thorough feedback! Really appreciate it. Will take a look and address.

[allthesignals]

@samselikoff pushed more changes. Please let me know if I can improve anything else.

[samselikoff]

Looks great – thank you Matt!