Document how to ignore code

[scottdotweb]

There isn't any documentation of how to ignore code for coverage. Namely, these directives:

• /* istanbul ignore <word>[non-word] [optional-docs] */
• /* istanbul ignore if */ and /* istanbul ignore else */
• /* istanbul ignore next */

I found a detailed guide in the IstanbulJS 1.0 repo. I was going to make a pull request to add it for feedback (presumably the content may need updating), but that repo has a license which says it's copyrighted by Yahoo. I'm happy to go ahead with the PR if the opinion is that's not a problem thanks to something in the project's history.