References newlines styleguide from dev docs

Currently, it's a bit hard to find because it's only referenced from the
contributing/style_guides. This commits adds it to our development
guidelines index

doc/development/README.md

@@ -133,6 +133,7 @@ from:
 - [Approval Rules](approval_rules.md)
 - [Feature categorization](feature_categorization/index.md)
 - [Wikis development guide](wikis.md)
+- [Newlines style guide](newlines_styleguide.md)
 
 ## Performance guides