Readme update.

README.md

@@ -20,6 +20,7 @@ Access to the `/dev/i2c` device files, which means either:
 
 ## Dependencies
 * libi2c-dev
+* libnvidia-ml-dev (if building with `USE_NVML=1`)
 
 ## Building
 `make`
@@ -27,7 +28,7 @@ Access to the `/dev/i2c` device files, which means either:
 ## Optional features
 
 ### NVML support
-Add the make flag `USE_NVML=1` and the it will also display the main GPU temperature ("GPU1") as reported by the NVIDIA driver.  It will also display the performance cap/clock reason.
+Add the make flag `USE_NVML=1` and the it will also display the main GPU temperature ("GPU1") as reported by the NVIDIA driver.  It will also display the performance cap/clock reason.  This requires NVML development files to be installed.
 
 ## Usage
 Note that when controlling fans directly through iCX3 they will fall offline from the Nvidia driver and show as 0 RPM until you return them to automatic mode.