New output plugin: logstash-output-documentdb

I would like to contribute a new logstash output plugin, logstash-output-documentdb. The plugin can be found here:
https://github.com/yokawasa/logstash-output-documentdb

The plugin makes possible to store logs in Azure DocumentDB, a managed NoSQL database service provided by Microsoft Azure. Please add the plugin to the logstash-plugins project if you find it useful

Start by reviewing the existing plugin in the linked logstash-output-documentdb repository and compare it with the conventions in the logstash-plugins project. Determine the required review and inclusion steps for an Azure DocumentDB output plugin; done means the plugin is accepted and added to the logstash-plugins project.

Written by the indexing model from the issue text.