Make it clear where the coverage artifacts are found

[joshuabremerdexcom]

Purpose

It took me a while to find where the coverage files existed. I was sure I had misconfigured it so i was reading through the README expecting it to tell me where to look for my files. What really happened was that my sidebar in VSCode was hiding the files that I had set in my .gitignore, so finally I checked the actual directory and found the folder.

I thought this Coverage Artifacts section could help.

Here's the final README: https://github.com/joshuabremerdexcom/ember-cli-code-coverage/blob/patch-1/README.md