Move style guide to CONTRIBUTING.md.

CONTRIBUTING file is promoted by GitHub, users see it with every issue.

This is just a proposal, you can close pull req if you like STYLEGUIDE more as more comprehensive name.

I think CONTRIBUTING will still be read by pull request submitters tho.
Fixed typo
Merge pull request #417 from chaplinjs/topics/remove-styleguide

Move style guide to CONTRIBUTING.md.