Revise README document about testing

[saschanaz]

Current the Testing section says:

Current test coverage, as documented in coverage.html, is 95%. You can run your own coverage analysis with:

jscoverage lib lib-cov

But I think we haven't used it for a while, as that line now throws because the old tool does not support ES2015+. We may just remove it, or replace it with modern tools if we actually want the coverage test. (See also mochajs/mocha#2356)

And...

Testing on IE and older versions will happen progressively.

Definitely no.

Thoughts? @marcoscaceres

[marcoscaceres]

Let's kill it. We should probably set up a newer coverage tool at some point tho.