It's basically Slimsy for Umbraco 9, so thanks to Jeavon.
This is a port of Slimsy, the core service, for Umbraco 9.0.1. It contains almost all the functionality, except for RTE support and a couple of arguments that is not supported anymore. You would use it the same way, as you are using Slimsy today. This port is made out of a combination of interest and boredome. Read below what it is not.
It is not a ready-to-go package. It's just the source code. Use it as it is, copy paste it to your project, or fork it and do something with it. It is NOT thoruoughly tested, but it does build. It is not 100% stand-alone, as it is a port of Slimsy, so beware of the lazysizes and picturefill scripts, or modify it yourself to work without it and suit your needs.
It basically works the same way as Slimsy. Check the UmbracoSrcSetGeneratorConfig.cs file to see configuration options you can override in the appconfig.json of your site.
As said, the operation is dependent on lazysizes and picturefill, as this is just a port of Slimsy.
The difference
Slimsy used an extension of the UrlHelper. This is a service that you can inject directly into your view. Just do @inject UmbracoSrcSetGeneratorService srcSetGenService or similar.
Don't expect much, if anything. But feel free to do with it whatever you want. Also, if the license file is filled out wrong, let me know, I am not trying to screw anyone out of their credits. Anyway...
Have fun!