docs: add documentdb-session to list of session stores

closes expressjs#348

Author: dwhieb

README.md

@@ -577,6 +577,11 @@ and other multi-core embedded devices).
 [connect-sqlite3-url]: https://www.npmjs.com/package/connect-sqlite3
 [connect-sqlite3-image]: https://img.shields.io/github/stars/rawberg/connect-sqlite3.svg?label=%E2%98%85
 
+[![★][documentdb-session-image] documentdb-session][documentdb-session-url] A session store for Microsoft Azure's [DocumentDB](https://azure.microsoft.com/en-us/services/documentdb/) NoSQL database service.
+
+[documentdb-session-url]: https://www.npmjs.com/package/documentdb-session
+[documentdb-session-image]: https://img.shields.io/github/stars/dwhieb/documentdb-session.svg?label=%E2%98%85
+
 [![★][express-nedb-session-image] express-nedb-session][express-nedb-session-url] A NeDB-based session store.
 
 [express-nedb-session-url]: https://www.npmjs.com/package/express-nedb-session