Add a goal section to the readme

[RunDevelopment]

When I first started this project, it was just a collection of simple rules I came up with to make reviewing regexes easier. But as the project grew and rules became more numerous, my focus started to shift. While the vague promise of "writing better regular expressions" may have been a good umbrella for what I've been doing until now, it doesn't describe the function and goals of the project at all.

Task:

• Describe in a few sentences in plain English what the purpose of this plugin is and how it tries to achieve its goals.
• List non-goals and make it clear that this plugin is opinionated.
• Be brief!

The section will be placed directly below the slogan, so people don't have to search for this information.