[Feature] document an example for updating multiple sitemaps

[derduher]

#312

[huntharo]

The related project below shows how to write up to hundreds of millions of items (tested) into sitemaps using AWS infrastructure (Lambda / Kinesis Streams / DynamoDB / S3):

https://github.com/shutterstock/streaming-sitemaps

The project can be used as a guide for how to perform this using other infrastructures. The problem is not simple, unfortunately.