Add notes on OpenCV debug libs

Compile-in-Windows.md

@@ -60,4 +60,7 @@ If Commit Staged button is greyed out:
 
 ## Notes
 
-MSVC compiles std::string incompatibly for Release and Debug builds, which means you can't mix different builds of libraries that include std::string in their APIs such as the UHD library. There's currently only a Release build of uhd.dll in the sdrangel-windows-externals repo. This will cause an exception if used with a Debug build of SDRangel. As a workaround, you'll need to either delete uhd.dll before debugging, or replace it with a Debug version you've built.
+MSVC compiles std::string incompatibly for Release and Debug builds, which means you can't mix different builds of libraries that include std::string in their APIs such as the UHD library. There's currently only a Release build of uhd.dll in the sdrangel-windows-externals repo. This will cause an exception if used with a Debug build of SDRangel. As a workaround, you'll need to either delete uhd.dll before debugging, or replace it with a Debug version you've built. 
+
+The debug libraries for OpenCV are too large for github (>100MB), so have been zipped. If doing a debug build, these files should be extracted in sdrangel\external\windows\opencv4\x64\vc16\bin
+