add missing debian/ubuntu pre-req to readme (#59)

cmake was missing for some reason
diff --git a/README.md b/README.md
index f3d0ab5..75ee141 100644
--- a/README.md
+++ b/README.md
@@ -7,7 +7,7 @@
 Linux/Mac: use your favorite package tool. For example, on Ubuntu:
 
 ```console
-sudo apt install build-essential pkg-config libusb-1.0-0-dev
+sudo apt install build-essential pkg-config libusb-1.0-0-dev cmake
 ```
 
 Windows: download from here https://libusb.info/