Expand README

Make it explicit in README that we build and locally install NVBench first,
and then build Python package use the library as a dependency.

The nvbench library is installed into Python layout alongside the native
extension.

Author: oleksandr-pavlyk

python/README.md

@@ -19,10 +19,18 @@ Now switch to python folder, configure and install NVBench library, and install
 cd nvbench/python
 cmake -B nvbench_build --preset nvbench-ci -S $(pwd)/.. -DCMAKE_CUDA_COMPILER=/usr/local/cuda/bin/nvcc -DNVBench_ENABLE_EXAMPLES=OFF -DCMAKE_INSTALL_PREFIX=$(pwd)/nvbench_install
 cmake --build nvbench_build/ --config Release --target install
+```
+
+### Build Python extension
+
+Specify location local installation of `NVBench` library and perform editable `pip install`:
 
+```
 nvbench_DIR=$(pwd)/nvbench_install/lib/cmake CUDACXX=/usr/local/cuda/bin/nvcc pip install -e .
 ```
 
+Note that `CUDACXX` must be set for NVBench cmake script to work, but Python extension itself only uses host compiler.
+
 ### Verify that package works
 
 ```