README: Add documentation for --led-no-busy-waiting

README.md

@@ -380,6 +380,20 @@ Use this also if you want to have a stable baseline refresh rate when using
 the vsync-multiple flag `-V` in the [led-image-viewer] or
 [video-viewer] utility programs.
 
+```
+--led-no-busy-waiting     : Don't use busy waiting when limiting refresh rate.
+```
+
+This allows to switch from busy waiting to sleep waiting when limiting the
+refresh rate (`--led-limit-refresh`).
+
+By default, refresh rate limiting uses busy waiting, which is CPU intensive but
+gives most accurate timings. This is fine for multi-core boards.
+
+On single core boards (e.g.: Raspberry Pi Zero) busy waiting makes the system
+unresponsive for other/background tasks. There, sleep waiting improves the
+system's responsiveness at the cost of slightly less accurate timings.
+
 ```
 --led-scan-mode=<0..1>    : 0 = progressive; 1 = interlaced (Default: 0).
 ```