clarify readme and explain start-order
diff --git a/README.md b/README.md
index 46531cb..cbc6f90 100644
--- a/README.md
+++ b/README.md
@@ -6,4 +6,16 @@
This module is part of the [Apache Sling](https://sling.apache.org) project.
-This module extends the Sling Featuremodel Launcher with [content package](https://jackrabbit.apache.org/filevault/index.html) capabilities. It allows to fill a [`FSPackageRegistry`](https://jackrabbit.apache.org/filevault/apidocs/org/apache/jackrabbit/vault/packaging/registry/impl/FSPackageRegistry.html) with precalculated execution plans (via OSGi configuration) which automatically get executed during repository start by [ExecutionPlanRepoInitializer](https://github.com/apache/sling-org-apache-sling-jcr-packageinit/blob/master/src/main/java/org/apache/sling/jcr/packageinit/impl/ExecutionPlanRepoInitializer.java)
+This module extends the Sling Featuremodel Launcher with [content package](https://jackrabbit.apache.org/filevault/index.html) capabilities. The format of the extension is described in [Extension Content-Packages](https://github.com/apache/sling-org-apache-sling-feature/blob/master/docs/extensions.md#built-in-extension-content-packages).
+
+To influence the package installation order the following format must be used
+
+```
+"content-packages:ARTIFACTS|required":[
+ "id":"org.apache.sling.myapp:my-content-package:zip:1.0.0"
+ "start-order": 1
+]
+```
+The default start order is the maximum integer value (i.e. the package is processed after all packages with an explicit start-order).
+
+All content-packages from the model are registered (externally) in a [`FSPackageRegistry`](https://jackrabbit.apache.org/filevault/apidocs/org/apache/jackrabbit/vault/packaging/registry/impl/FSPackageRegistry.html) and scheduled for installation with execution plans (given via OSGi configuration) which automatically get executed during repository start by [ExecutionPlanRepoInitializer](https://github.com/apache/sling-org-apache-sling-jcr-packageinit/blob/master/src/main/java/org/apache/sling/jcr/packageinit/impl/ExecutionPlanRepoInitializer.java)
