Document a PATH problem on Windows.

This showed up in #220 and #327.

Author: nnethercote

guide/src/guide/getting_started.md

@@ -276,12 +276,24 @@ There are two ways to build and run this example: natively, and with docker.
 
 ### Native
 
-If you have all the required libraries installed, try building with `cargo build`. If you get an
-error "libnvvm.so.4: cannot open shared object file", you will need to adjust `LD_LIBRARY_PATH`,
-something like this:
+If you have all the required libraries installed, try building with `cargo build`.
+
+If you get an error "libnvvm.so.4: cannot open shared object file", you will need to adjust
+`LD_LIBRARY_PATH`, something like this:
 ```
 export LD_LIBRARY_PATH="/usr/local/cuda/nvvm/lib64:${LD_LIBRARY_PATH}"
 ```
+
+If you get an error "error: couldn't load codegen backend" on Windows, you will need to adjust
+`PATH`, something like this with CUDA 12:
+```
+$env:PATH += ";C:\Program Files\NVIDIA GPU Computing Toolkit\CUDA\v12.9\nvvm\bin"
+```
+or this with CUDA 13:
+```
+$env:PATH += ";C:\Program Files\NVIDIA GPU Computing Toolkit\CUDA\v12.9\nvvm\bin\x64"
+```
+
 You should then be able to `cargo run`, and see the expected output:
 ```
 c = [3.0, 5.0, 7.0, 9.0]