Re-add command to install vcpkg

`vcpkg integrate install` must be executed so that msbuild will automatically install external dependencies.

It was removed in https://github.com/bitcoin/bitcoin/commit/712f95d3324d02310dd468e7bfd1e1b0df432e77

It was originally added in https://github.com/bitcoin/bitcoin/commit/76445677586a4c2fa72606b662269a4390c2e71f

build_msvc/README.md

@@ -27,7 +27,11 @@ Options for installing the dependencies in a Visual Studio compatible manner are
 - Download the source code, build each dependency, add the required include paths, link libraries and binary tools to the Visual Studio project files.
 - Use [nuget](https://www.nuget.org/) packages with the understanding that any binary files have been compiled by an untrusted third party.
 
-The [external dependencies](https://github.com/bitcoin/bitcoin/blob/master/doc/dependencies.md) required for building are listed in the `build_msvc/vcpkg.json` file. The `msbuild` project files are configured to automatically install the `vcpkg` dependencies.
+The [external dependencies](https://github.com/bitcoin/bitcoin/blob/master/doc/dependencies.md) required for building are listed in the `build_msvc/vcpkg.json` file. To ensure `msbuild` project files automatically install the `vcpkg` dependencies use:
+
+```
+vcpkg integrate install
+```
 
 Qt
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