Nested Operations docs should be updated

[sambostock]

We include the following in README.md:

What do I do if each iteration takes a long time, because it's doing nested operations? If your each_iteration is complex, we recommend enqueuing another job, which will run your nested business logic. We may expose primitives in the future to do this more effectively, but this is not terribly common today. We recommend to read goo.gl/UobaaU to learn more about nested operations.

The link we provide is problematic for two reasons:

• it links to our internal Discourse instance, despite this being a public repository
• the answer explicitly directs the user to use BackgroundQueue.enqueue (as well as start_id: and last_id:) which is Shopify/shopify specific, even though JobIteration is used outside that repo

We should link to something public and non-Shopify specific.