[DOCS] Introduce the `vendor-shim` blueprint
[DOCS] Introduce the `vendor-shim` blueprint

Documents [#4888].

There isn't much documentation around how to properly wrap an otherwise
global `window` dependency in an ES6 compliant module.

The generated blueprint will nudge consumers in the correct direction.

Insprired by [this stack overflow response][shim].

[#4888]: https://github.com/ember-cli/ember-cli/pull/4888
[shim]: http://stackoverflow.com/questions/30443351/import-dependencies-in-ember-cli-e-g-import-math-js/30584671#30584671

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