Update BUILD_WIN.md Qt instructions.

BUILD_WIN.md

@@ -53,7 +53,26 @@ Download the file named win64-x64 Installer from the [CMake Website](https://cma
 
 Install version 10.15.0 LTS (or greater) of [Node.JS and NPM](<https://nodejs.org/en/download/>).
 
-### Step 5. Create VCPKG environment variable
+### Step 5. Qt
+
+Install version 5.12.3 of [Qt](<https://www.qt.io/download-open-source>), as well as the following packages:
+* Qt 5.12.3
+* MSVC 2017 64-bit
+* Qt WebEngine
+* Qt Script (Deprecated)
+
+You may also want the Qt Debug Information Files and Sources packages, for convenience.
+
+You'll need to create the environment variable that CMake uses to find your system's Qt install.
+
+To create this variable:
+* Navigate to 'Edit the System Environment Variables' through the start menu.
+* Click on 'Environment Variables'
+* Select 'New'
+* Set "Variable name" to `QT_CMAKE_PREFIX_PATH`
+* Set "Variable value" to %QT_INSTALL_DIR%\5.12.3\msvc2017_64\lib\cmake, where %QT_INSTALL_DIR% is the directory you specified for Qt's installation. The default is C:\Qt.
+
+### Step 6. Create VCPKG environment variable
 In the next step, you will use CMake to build Vircadia. By default, the CMake process builds dependency files in Windows' `%TEMP%` directory, which is periodically cleared by the operating system. To prevent you from having to re-build the dependencies in the event that Windows clears that directory, we recommend that you create a `HIFI_VCPKG_BASE` environment variable linked to a directory somewhere on your machine. That directory will contain all dependency files until you manually remove them.
 
 To create this variable: