Document new agg types in README

[jameslamb]

This project's main README.md contains a list of aggregations supported by uptasticsearch. That list is 2+ years old at this point, and was generated based on older versions of Elasticsearch.

To close this issue, ensure that any aggregations made available in ES5, ES6, and ES7 are listed in that table. If any are found which do now exist across all of ES 1.x, 2.x, 5.x, 6.x, and 7.x, indicate which versions they are supported for.

For any that are not currently supported in uptasticsearch, create additional issues here which document the need to implement support for them in this project.