Convert plain text path docs/styleguide.md to a Markdown link docs/styleguide.md so it’s clickable in viewers. (#226)

Co-authored-by: Felix Kimmerle <[email protected]>

Author: FelixKimmerle

CONTRIBUTING.md

@@ -13,7 +13,7 @@ We appreciate all kinds of contributions to this project. This document gives yo
 
 ## Style
 
-Please follow the guidelines in `docs/styleguide.md`.
+Please follow the guidelines in [docs/styleguide.md](docs/styleguide.md).
 
 ## Licensing of Contributed Material