Add example of graceful degradation

[Malvoz]

it's worth making an example, if we can, of the built-in that falls back to an image map in Safari.

Originally posted by @prushforth in #39 (comment)

[Malvoz]

There's an example in index-map-area.html. We can copy & paste relevant parts and add to high-level-api.md#graceful-degradation-and-progressive-enhancement. I've tested it in other browsers that don't support built-ins such as IE 10, but it'd be great if someone can confirm the fallback works correctly in Safari as well.

[prushforth]

I've been working on recovering this functionality in web-map-custom-element. Unfortunately a particular part of the original code ported from the polymer project was not covered by tests and there was a regression due to work done this past winter. I have it fixed locally and am planning to add tests to prevent this from happening again, will probably get to it this week or so.