add README for nvtxw project

src/main/cpp/profiler/README-nvtxw.txt

@@ -0,0 +1,22 @@
+1. NvtxwEvents.h, NvtxwEvents.cpp are copied from Nsight Systems source code. They need to be kept in sync between this project and Nsight Systems.
+
+2. Need to set the NVTXW_BACKEND environment variable for the libNvtxwBackend.so library in the host directory a current build of Nsight Systems. For example:
+   > export NVTXW_BACKEND=/opt/nvidia/nsight-systems/2024.6.0/host-linux-x64/libNvtxwBackend.so
+
+3. Run like this:
+      > ./target/jni/cmake-build/profiler/spark_rapids_profile_converter  -w -o file3021460.nsys-rep [email protected]
+   and get output similar to this:
+      Backend implementation loaded!  Applying config string...
+      Loader config key/value pairs not provided
+      Creating report: "file3021460.nsys-rep"
+      - Created session: file3021460
+      Session config key/value pairs not provided
+      - Created stream: Stream1
+         Domain: SparkRAPIDS
+         Scope: 
+      - Destroyed stream: Stream1
+      3946 events imported
+      - Destroyed session: file3021460
+      Backend implementation prepared for unload.
+
+4. Load into nsight systems UI: nsys-ui file3021460.nsys-rep