readme: add HDR and gamescope utility sections

[matte-schwartz]

Leaving this as a draft for now pending:

• [ ] fill in more information for gamescopereaper and gamescopestream after testing
• [ ] verify details of all utility descriptions
• [ ] check for any stylistic changes

what I'm undecided on still:

1. should we add more information regarding which DEs currently support HDR w/ nested gamescope?
2. should we include -Dforce_fallback_for in the readme build instructions to avoid confusion? it would get rid of any initial build errors for people just copying the website build instructions. I've included this already in the initial draft.

feel free to close if you've got something going locally already, but I think this covers most of the changes that I'm aware of in the past few months.