Add Graph database section

README.md

@@ -1136,3 +1136,31 @@ Wide column stores offer high availability and high scalability.  They are often
 * [Bigtable architecture](http://www.read.seas.harvard.edu/~kohler/class/cs239-w08/chang06bigtable.pdf)
 * [HBase architecture](https://www.mapr.com/blog/in-depth-look-hbase-architecture)
 * [Cassandra architecture](http://docs.datastax.com/en/archived/cassandra/2.0/cassandra/architecture/architectureIntro_c.html)
+
+#### Graph database
+
+<p align="center">
+  <img src="http://i.imgur.com/fNcl65g.png">
+  <br/>
+  <i><a href=https://en.wikipedia.org/wiki/File:GraphDatabase_PropertyGraph.png>Source: Graph database</a></i>
+</p>
+
+> Abstraction: graph
+
+In a graph database, each node is a record and each arc is a relationship between two nodes.  Graph databases are optimized to represent complex relationships with many foreign keys or many-to-many relationships.
+
+Graphs databases offer high performance for data models with complex relationships, such as a social network.  They are relatively new and are not yet widely-used; it might be more difficult to find development tools and resources.  Many graphs can only be accessed with [REST APIs](#representational-state-transfer-rest).
+
+##### Source(s) and further reading: graph
+
+* [Graph database](https://en.wikipedia.org/wiki/Graph_database)
+* [Neo4j](https://neo4j.com/)
+* [FlockDB](https://blog.twitter.com/2010/introducing-flockdb)
+
+#### Source(s) and further reading: NoSQL
+
+* [Explanation of base terminology](http://stackoverflow.com/questions/3342497/explanation-of-base-terminology)
+* [NoSQL databases a survey and decision guidance](https://medium.com/baqend-blog/nosql-databases-a-survey-and-decision-guidance-ea7823a822d#.wskogqenq)
+* [Scalability](http://www.lecloud.net/post/7994751381/scalability-for-dummies-part-2-database)
+* [Introduction to NoSQL](https://www.youtube.com/watch?v=qI_g07C_Q5I)
+* [NoSQL patterns](http://horicky.blogspot.com/2009/11/nosql-patterns.html)