[docs] add section to configuration about working in monorepos

[hmajoros]

running ember install ember-cli-typescript will automatically add ember-cli-typescript as a dependency in your top level package.json, but in a monorepo you may need to additionally add ember-cli-typescript into the top level package for any apps in your monorepo. Without this step, you will never kick off typechecking as part of the build because this conditional will never be met:

https://github.com/typed-ember/ember-cli-typescript/blob/22abe759e1bdea79d06afe2c0e77f7e10efcf055/ts/addon.ts#L31

We should add a note to the docs somewhere (likely during the installation/configuration?) to remind folks working in a mono-repo that they'll need to add ember-cli-typescript as a devDependency in the top level package.json of any app inside their mono-repo.