Clarify Progress Feedback guideline

[JanMiksovsky]

Breaking out this feedback from @arkihillel in #10:

Progress Feedback — For long operations, does the component provide appropriate feedback?

Long operations should only be allowed whenever the network is used (think API calls or lazy loading external resources). If an UI element does need computations in order to work, it should be coupled with an external, functional, element.

e.g. a Fibonacci calculator should be composed of two elements <fib-view> and <fib-calc>. Each one should have its own set of events and the user should be in control of whether or not to show a loader and what this loader would look like. Then again, this concern may already be solved by letting the user customize the calculator loader styling

I agree that such factoring is a good idea. I'm not sure how much detail to provide at this point on strategies for doing so, as we're still figuring out the best techniques.

In the meantime, I've tweaked the wording of the Progress Feedback guideline a bit, changing "does the component" to "can the component", and adding a little more content. Let me know if that helps address your point.