[README] Add Why

Author: Vescrity

README.md

@@ -10,3 +10,7 @@ dmald <latency_value> #(or -1 to terminate daemon)
 
 - The first process will be the daemon, and others will send signal to the daemon to change the value.  
 - You may need to be **root** or a member of **realtime**.
+
+## Why
+
+Some DAW like Ardour built-in the function of changing latency of CPU DMA, but hard to find a simple tool to set the latency when I want to run some other standalone application (e.g. Carla) and want to avoid too many XRUNs.