Expand ROOT_USE_FILE

chapters/packages/ROOT.md

@@ -11,9 +11,23 @@ ROOT supports config file discovery, so you can just do:
 
 to attempt to find ROOT. If you don't have your paths set up, you can pass `-DROOT_DIR=$ROOTSYS/cmake` to find ROOT. (But, really, you should source `thisroot.sh`)
 
-## The wrong way
+## The simple but wrong way
 
-ROOT provides a utility to set up a ROOT project, which you can activate using `include(${ROOT_USE_FILE})`. This will automatically make ugly global variables for you. It will save you a little time setting up, and will waste massive amounts of time later when you try to do anything more complicated.  Don't use it.
+ROOT provides a utility to set up a ROOT project, which you can activate using `include(${ROOT_USE_FILE})`. This will automatically make ugly global variables for you. It will save you a little time setting up, and will waste massive amounts of time later if you try to do anything tricky. As long as you aren't making a library, it's probably fine for simple scripts. Includes and flags are set globally, but you'll still need to link to `${ROOT_LIBRARIES}` yourself.
+
+Here's what it would look like:
+
+```cmake
+cmake_minimum_required(VERSION 3.11)
+
+project(Simple LANGUAGES CXX)
+
+find_package(ROOT CONFIG REQUIRED COMPONENTS Minuit)
+include("${ROOT_USE_FILE}")
+
+add_executable(simple simple.cxx simple.h)
+target_link_libraries(simple ${ROOT_LIBRARIES})
+```
 
 ## The right way (Targets)