Fix instructions on building cub

.gitignore

@@ -1,3 +1,4 @@
 .p4config
 *~
 \#*
+/build

README.md

@@ -143,23 +143,29 @@ See the [changelog](CHANGELOG.md) for details about specific releases.
 <br><hr>
 <h3>Development Process</h3>
 
+CUB and thrust depends on each other. To build CUB, you need to clone the
+same version of thrust.
+
 CUB uses the [CMake build system](https://cmake.org/) to build unit tests,
 examples, and header tests. To build CUB as a developer, the following
 recipe should be followed:
 
 ```
 # Clone CUB repo from github:
 git clone https://github.com/NVIDIA/cub.git
+# Clone the same version of thrust:
+git clone --recursive https://github.com/NVIDIA/thrust.git /path/to/thrust_repo
+
 cd cub
 
 # Create build directory:
 mkdir build
 cd build
 
 # Configure -- use one of the following:
-cmake ..   # Command line interface.
-ccmake ..  # ncurses GUI (Linux only)
-cmake-gui  # Graphical UI, set source/build directories in the app
+cmake -DThrust_DIR=/path/to/thrust_repo/thrust/cmake ..   # Command line interface.
+ccmake -DThrust_DIR=/path/to/thrust_repo/thrust/cmake ..  # ncurses GUI (Linux only)
+cmake-gui  # Graphical UI, set source/build/thrust directories in the app
 
 # Build:
 cmake --build . -j <num jobs>   # invokes make (or ninja, etc)