add link to styleguide

Author: keithamus

CONTRIBUTING.md

@@ -11,6 +11,7 @@ Contributions are always welcome! Before contributing, please read the
   [good first issue label](https://github.com/WebComponentsGuide/webcomponents.guide/labels/good%20first%20issue).
 - If you're contributing to larger pieces, like an idea or new section, tutorial, or blog post, then please raise an
   issue first! This way we can discuss an action plan and figure out a high level overview of what we should write.
+- [Reading the styleguide](./STYLEGUIDE.md) will help you write content that fits with our intended style.
 
 ### Setup