Merge branch 'dz-update-permissions' into 'master'

Update permissions.md with guidance for developing new features

See merge request gitlab-org/gitlab-ce!18344

doc/user/permissions.md

@@ -15,6 +15,10 @@ GitLab [administrators](../README.md#administrator-documentation) receive all pe
 To add or import a user, you can follow the
 [project members documentation](../user/project/members/index.md).
 
+## Principles behind permissions
+
+See our [product handbook on permissions](https://about.gitlab.com/handbook/product#permissions-in-gitlab)
+
 ## Project members permissions
 
 The following table depicts the various user permission levels in a project.