Add specific instructions for addons in README

[rajasegar]

We may require specific instructions on how to use this blueprint with and without custom options.

Using the blueprint with addons

$ ember addon my-ember-addon
$ cd my-ember-addon
$ ember init -b ember-a11y-blueprint

Using custom language for the blueprint with addons

$ ember addon my-ember-addon
$ cd my-ember-addon
$ ember init -b ember-a11y-blueprint --lang=es

[kellyselden]

Do addons need special instructions compared to apps? I don't see the need.

[rajasegar]

I have a doubt here then, why are we creating an addon and then applying the blueprint with init. Can't we create an addon with the blueprint in the first place itself?

$ ember addon my-ember-addon -b ember-a11y-blueprint

[kellyselden]

This is intended to be a partial blueprint as described by emberjs/rfcs#477. It "decorates" a normal app blueprint with extras. This is why the blueprint is mostly empty.

[kellyselden]

There may be a better way to "inherit" the default index.html without keeping a complete copy in sync.

[rajasegar]

I personally prefer both types of blueprints, one full-blown blueprint for starting new ember projects with first-class a11y support and one partial blueprint to update existing apps to support recommended a11y standards. And if we figure out the best way to inherit the default blueprint I think we can solve both the above use-cases.