add section to README outlining pros/cons of using sorri over lorri

[adrian-gierakowski]

some points I'd be interested in:

• support for env vars set in shellHook: lorri [x], sorri [?]
• support for project local NIX_CONF_DIR set in .envrc: lorri [?], sorri [?]

[adrian-gierakowski]

btw. I'm looking forward to try this out as it makes for one less thing to install globally

I guess this would make it to the pros list :)

[bbigras]

Please also compare against nix-direnv.