help/colors: add break before paragraph about colorscheme includes

Make it a well-visible subsection of the "Creating a Colorscheme"
section.

runtime/help/colors.md

@@ -236,6 +236,9 @@ Here's a list of subgroups used in micro's built-in syntax files.
 * type.keyword (If you want a special highlight for keywords like `private`)
 
 In the future, plugins may also be able to use color groups for styling.
+
+---
+
 Last but not least it's even possible to use `include` followed by the
 colorscheme name as string to include a different colorscheme within a new one.
 Additionally the groups can then be extended or overwritten. The `default.micro`