Add instructions for WinDbg Preview.

As per https://docs.microsoft.com/en-us/windows-hardware/drivers/debugger/loading-debugger-extension-dlls

We could also remove the winext directory suggestion, since this will
work for both "legacy" WinDbg and WinDbg Preview.

docs/README.md

@@ -15,6 +15,8 @@ JsDbg currently supports WinDbg and the Visual Studio debugger on Windows, and G
 
 2. Copy jsdbg.dll into the `winext` folder located next your `windbg.exe` installation. Make sure to use the x64 version of the dll for 64-bit WinDbg, and the x86 version for 32-bit WinDbg.
 
+3. To use JsDbg with [WinDbg Preview](https://docs.microsoft.com/en-us/windows-hardware/drivers/debugger/debugging-using-windbg-preview), the easiest way is to set the environment variable `_NT_DEBUGGER_EXTENSION_PATH` to the directory you extracted WinDbg to. This will also work for non-preview WinDbg.
+
 ### Usage
 
 1. Attach WinDbg to a running process or memory dump of Microsoft Edge/Chromium.