Extensive, nicely formatted guide for the new Shortcuts app, published directly by Apple today.
If you're absolutely new to the Shortcuts app and never played with Workflow before, this is where I'd start. The guide goes into great detail about what shortcuts are, how you can organize them, and what types of input are supported in Shortcuts. There's also an Advanced section where – and I never thought I'd see this – Apple explains x-callback-url and web APIs. Some of these sections have been adapted from the old Workflow documentation, but it's a fantastic resource regardless. And once you're done reading through Apple's materials, I'll be waiting here.