Document c3 integration with existing unit tests.

I am using C3 in a project that uses mocha and jsdom for unit testing. An issue I am getting is when a test runs through c3 code. At the moment, the main issue is `MutationObserver` is not supported. It would nice if you could document a suggestive approach of using c3 with existing unit tests.

My workaround at the moment is to stub out the `generate` function and return an empty div.