Update README.md

README.md

@@ -29,7 +29,10 @@ apt install qt5-default
 ```
 
 To build and install QtAV follow the steps on https://github.com/wang-bin/QtAV/wiki/Build-QtAV.
-Please build QtAV with cmake instead of qmake.
+Please build QtAV with cmake instead of qmake. Because the cmake build is using ``ninja``, you have to ensure it is installed:
+```
+apt install ninja
+```
 
 ## Building VanadiumCast
 ```