Find README recursively from cwd to base path

This changes how the README file is found in a way which hopefully doesn't stomp on your use case while being more flexible for a range of project structures.

Instead of assuming the README will be in `config.root` it recurses from `config.cwd` up to the Grunt base path until it finds a README file using a case insensitive glob that matches common file extensions for READMEs (including none at all).

This is useful for those of us with our Sass assets nested that want to keep the styleguide's README separate from the project's own README, but don't want the styleguide's README ending up in the middle directory. For my project, I need it to pick up `./app/styles/README.md` instead of `./app/README.md`. However, if no README exists, it still creates the the boilerplate in `config.root`.