SLING-5024 add/update readme

git-svn-id: https://svn.apache.org/repos/asf/sling/trunk@1703067 13f79535-47bb-0310-9956-ffa450edef68
diff --git a/README.txt b/README.txt
new file mode 100644
index 0000000..2c101be
--- /dev/null
+++ b/README.txt
@@ -0,0 +1,6 @@
+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:
+https://svn.apache.org/repos/asf/sling/trunk/contrib/nosql/mongodb-resourceprovider
+
+Please note that both implementations are not interchangeable because they store the data in MongoDB in different formats.