Add editor support to the readme

In this pull request i would like to add the supported text editors and plugins for simplecov.

I recently developed a [Atom Editor](http://atom.io) Package which displays the simplecov results in a graphical layout.

I think thats its a good idea to list the supported text editors in the README. If you know more supported editors and plugins, please comment and i will add it in this pull request.

#### Packages
- [Atom Editor: coverage](github.com/philipgiuliani/coverage)