Merge pull request #14 from phpcr/added_references

Added some references

Author: dbu

documentation/index.md

@@ -9,13 +9,22 @@ title: Documentation
 
 ## Reference
 
-- [API Reference](/doc/html/index.html)
+- [API Reference](/doc/html/index.html) - Online API documentation for the PHPCR project.
+- [Node Type CND Reference (Jackrabbit)](http://jackrabbit.apache.org/node-type-notation.html) - Detailed definition and
+  explanation of the CND node type format.
+- [JCR Query Cheatsheet (Magnolia)](https://wiki.magnolia-cms.com/display/WIKI/JCR+Query+Cheat+Sheet) - Examples of
+  the JCR2 query language.
 
 ## Slides
 
 - [PHPCR Introduction by David Buchmann and Lukas Smith](/slides.html)
 - [PHPCR, Une Introduction by Daniel Leech (in French)](http://phpcrtalk.dantleech.com)
 
+## Articles
+
+- [JCR or RDBMS - Why, when, how?](https://dev.day.com/content/ddc/blog/2009/01/jcrrdbmsreport/_jcr_content/images/jcrrdbmsreport/jcr_rdbms_report_chapuis.pdf)
+- [Davids Model: A guide for content modeling](http://wiki.apache.org/jackrabbit/DavidsModel)
+
 ## Other
 
 - [PHPCR Shell](/documentation/phpcr-shell)