Update documentation how to build for Windows

Change-Id: I41d8b6a796820dfa0fba89389ef15a8915e666f8
diff --git a/README.md b/README.md
index e870ed9..5840bfb 100644
--- a/README.md
+++ b/README.md
@@ -70,8 +70,12 @@
 For Windows Ninja has been verified to work well, but any of the supported
 generators should be possible to use.
 
+Please note that the maximum path length is by default set to 260 characters on
+Windows. It is recommended to place the Ethos-U source tree and build folder at
+the root of the file system.
+
 ```
-CMD> cmake -G Ninja -B build targets/corstone-300 -DCMAKE_TOOLCHAIN_FILE=%CD%\cmake\toolchain\arm-none-eabi-gcc.cmake
+CMD> cmake -G Ninja -B build targets/corstone-300 --toolchain %CD%\cmake\toolchain\arm-none-eabi-gcc.cmake
 CMD> cmake --build build
 ```