Fix non-apply of locale parameter to faker.js

[alkihis]

In 1.8.0, the ability of specifiying the locale used in faker.js has been implemented, but doesn't apply automatically (and isn't documented).

I've haven't managed to make it work with the given example:

for (const fixture of fixturesIterator(fixtures)) {
  const entity = await builder.build(fixture);
  await getRepository(entity.constructor.name).save(entity);
}

with the given fixture:

entity: User
locale: fr
items:
  user{1..10}:
    firstName: '{{name.firstName}}'
    lastName: '{{name.lastName}}'
    birthDate: '{{date.past}}'
    email: '{{internet.email}}'
    phone: '{{phone.phoneNumber}}'
    country: France
    city: '{{address.city}}'

I've added the line fixture.locale = 'fr'; before builder.build line, and it works fine (so I've assumed locale parameter from yml file wasn't read properly).

This PR fixes the missing locale property of a IFixture, and add documentation in README.md to apply a locale for faker.js.

[RobinCK]

tests are failing