Add a comparison to Refactoring Essentials

Refactoring Essentials (http://vsrefactoringessentials.com/) is a tool which has the same purpose as Roslynator. It has existed for much longer so it would be useful to see how Roslynator is superior. A simple list in the documentation which consists of what Roslynator has and Refactoring Essentials doesn't, and vice-versa, would help users make their choice and therefore increase adoption of Roslynator.