readme: add a note about PATH

README.md

@@ -56,6 +56,8 @@ i3status-rust hueshift block has the builtin support for this backend since 0.21
     }
 ```
 
+Note: make sure `wl-gammarelay-rs` is in your `PATH` at the time `waybar` is launched, or use the full path to the binary.
+
 ## Watch for changes
 
 You can run `wl-gammarelay-rs watch <format>` to watch for changes. Each time some property changes, a new line will be printed based on <format> string. `{t}` expands into temperature, `{b}` into brightness (0 to 1) and `{bp}` expands into brightness in percents (0 to 100).