README: document how to choose between libusb & hidraw backends

Author: Be-ing

README.md

@@ -45,7 +45,10 @@ HIDAPI has five back-ends:
 * Mac (using IOHidManager)
 
 On Linux, either the hidraw or the libusb back-end can be used. There are
-tradeoffs, and the functionality supported is slightly different.
+tradeoffs, and the functionality supported is slightly different. Both are
+built by default. It is up to the application linking to hidapi to choose
+the backend at link time by linking to either `libhidapi-libusb` or
+`libhidapi-hidraw`.
 
 __Linux/hidraw__ (`linux/hid.c`):