Note regarding absolute paths (prusa3d#6111)

doc/How to build - Windows.md

@@ -38,6 +38,8 @@ cd build
 cmake .. -G "Visual Studio 16 2019" -DCMAKE_PREFIX_PATH="c:\src\PrusaSlicer-deps\usr\local"
 ```
 
+Note that `CMAKE_PREFIX_PATH` must be absolute path. A relative path like "..\..\PrusaSlicer-deps\usr\local" does not work.
+
 ### Compile PrusaSlicer. 
 
 Double-click c:\src\PrusaSlicer\build\PrusaSlicer.sln to open in Visual Studio 2019.
@@ -175,4 +177,4 @@ option to CMake, this will only produce a _Release_ build.
 Refer to the CMake scripts inside the `deps` directory to see which dependencies are built in what versions and how this is done.
 
 \*) Specifically, the problem arises when building boost. Boost build tool appends all build options into paths of
-intermediate files, which are not handled correctly by either `b2.exe` or possibly `ninja` (?).
+intermediate files, which are not handled correctly by either `b2.exe` or possibly `ninja` (?).