[docs] Add VCPKG_CHAINLOAD_TOOLCHAIN_FILE to the Triplets documentation.

docs/users/triplets.md

@@ -31,13 +31,20 @@ Valid options are `WindowsStore` or empty. Empty corresponds to Windows Desktop
 When setting this variable to `WindowsStore`, you must also set `VCPKG_CMAKE_SYSTEM_VERSION` to `10.0`.
 
 ### VCPKG_PLATFORM_TOOLSET
-Specifies the C/C++ compiler toolchain to use.
+Specifies the VS-based C/C++ compiler toolchain to use.
 
 This can be set to `v141`, `v140`, or left blank. If left blank, we select the latest compiler toolset available on your machine.
 
 Visual Studio 2015 platform toolset is `v140`  
 Visual Studio 2017 platform toolset is `v141`
 
+### VCPKG_CHAINLOAD_TOOLCHAIN_FILE
+Specifies an alternate CMake Toolchain file to use.
+
+This (if set) will override all other compiler detection logic. By default, a toolchain file is selected from `scripts/toolchains/` appropriate to the platform.
+
+See also the CMake documentation for toolchain files: https://cmake.org/cmake/help/v3.11/manual/cmake-toolchains.7.html.
+
 ## Per-port customization
 The CMake Macro `PORT` will be set when interpreting the triplet file and can be used to change settings (such as `VCPKG_LIBRARY_LINKAGE`) on a per-port basis.