Added new NoSQL resource (#5948)

* Added new NoSQL resource

* Update books/free-programming-books-langs.md

Co-authored-by: David Ordás <3125580+davorpa@users.noreply.github.com>

* Update free-programming-books-langs.md

Co-authored-by: David Ordás <3125580+davorpa@users.noreply.github.com>

books/free-programming-books-langs.md

@@ -1538,6 +1538,7 @@ That section got so big, we decided to split it into its own file, the [BY SUBJE
 * [CouchDB: The Definitive Guide](http://guide.couchdb.org)
 * [Extracting Data from NoSQL Databases: A Step towards Interactive Visual Analysis of NoSQL Data](http://publications.lib.chalmers.se/records/fulltext/155048.pdf) - Petter Nasholm (PDF)
 * [Graph Databases](http://graphdatabases.com)
+* [How To Manage a Redis Database](https://www.digitalocean.com/community/books/how-to-manage-a-redis-database-ebook) - Mark Drake (PDF, EPUB)
 * [NoSQL Databases](http://www.christof-strauch.de/nosqldbs.pdf) - Christof Strauch (PDF)
 * [The Little Redis Book](http://openmymind.net/2012/1/23/The-Little-Redis-Book/) - K. Seguin (PDF, Epub)