Add Key-value store section

README.md

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 * **Eventual consistency** - the system will become consistent over a period of time, given that the system doesn't receive input during that period.
 
 In addition to choosing between [SQL or NoSQL](#sql-or-nosql), it is helpful to understand which type of NoSQL database best fits your use case(s).  We'll review **key-value stores**, **document-stores**, **wide column stores**, and **graph databases** in the next section.
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+#### Key-value store
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+> Abstraction: hash table
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+A key-value store generally allows for O(1) reads and writes and is often backed by memory or SSD.  Data stores can maintain keys in [lexicographic order](https://en.wikipedia.org/wiki/Lexicographical_order), allowing efficient retrieval of key ranges.  Key-value stores can allow for storing of metadata with a value.
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+Key-value stores provide high performance and are often used for simple data models or for rapidly-changing data, such as an in-memory cache layer.  Since they offer only a limited set of operations, complexity is shifted to the application layer if additional operations are needed.
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+A key-value store is the basis for more complex system systems such as a document store, and in some cases, a graph database.
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+##### Source(s) and further reading: key-value store
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+* [Key-value database](https://en.wikipedia.org/wiki/Key-value_database)
+* [Disadvantages of key-value stores](http://stackoverflow.com/questions/4056093/what-are-the-disadvantages-of-using-a-key-value-table-over-nullable-columns-or)
+* [Redis architecture](http://qnimate.com/overview-of-redis-architecture/)
+* [Memcached architecture](https://www.adayinthelifeof.nl/2011/02/06/memcache-internals/)