SLING-7167 Adjust READMEs

add uniform header linking to Sling project

git-svn-id: https://svn.apache.org/repos/asf/sling/trunk@1810902 13f79535-47bb-0310-9956-ffa450edef68
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+# Apache Sling MongoDB Resource Provider
+
+This module is part of the [Apache Sling](https://sling.apache.org) project.
+
 This is a Sling Resource Provider for MongoDB implementation (from 2012)
 
 The Sling provides also a different implementation based on the Generic NoSQL Resource Provider for MongoDB: