The readme needs to be improved

To setup vkbasalt properly the user needs to give the application (e.g. Steam) reading rights to the shader directory and access to the vkBasalt.conf.
This should be better comunicated in the README.md

Solutions would be running commands like:
```shell
flatpak override com.valvesoftware.Steam --filesystem=~/.local/share/reshade
flatpak override com.valvesoftware.Steam --filesystem=xdg-config/vkBasalt:ro
```
Or adding those entries via Flatseal:

Maybe there are better solutions to make vkbasalt work but this is what I had to do with steam.
Other applications may have already reading rights to the home directory but steam has some issues when given access to
```home:ro```

Start by reading README.md and checking its setup instructions against the reported Steam and Flatpak permissions. Done means documenting the required shader-directory and vkBasalt.conf access, including the shown override commands or Flatseal steps, while noting the home:ro limitation.

Written by the indexing model from the issue text.