docs(CONTRIBUTING): add link to the styleguide

Author: akinomyoga

CONTRIBUTING.md

@@ -42,8 +42,9 @@ that. Don't be disappointed if it does or doesn't happen instantly.
 
 Also, please bear the following coding guidelines in mind:
 
-- See the [API and naming](doc/api-and-naming.md) document for information
-  about conventions to follow related to those topics.
+- See the related documents, [API and naming](doc/api-and-naming.md) and
+  [Coding style guide](doc/styleguide.md), for information about conventions to
+  follow related to those topics.
 
 - Do not use Perl, Ruby, Python etc. to do text processing unless the
   command for which you are writing the completion code implies the