Add link to mental model

Signed-off-by: Marek Kubica <marek@tarides.com>

doc/explanation/package-management.md

@@ -164,7 +164,10 @@ access any package repositories.
 ## Building
 
 When building, Dune will read the information from the lock directory and set
-up build rules to use the packages. The rules include
+up rules to use the packages. Check {doc}`/explanation/mental-model` for
+details about rules.
+
+The rules that the package management sets up include:
 
 * Fetch rules to download the sources as well as any additional sources like
   patches and unpack them