[docs] Add config-environment.md

docs/users/config-environment.md

@@ -0,0 +1,42 @@
+## Environment and Configuration
+
+### Environment Variables
+
+#### VCPKG_DOWNLOADS
+
+This environment variable can be set to an existing directory to use for storing downloads instead of the internal
+`downloads/` directory. It should always be set to an absolute path.
+
+#### VCPKG_FEATURE_FLAGS
+
+This environment variable can be set to a comma-separated list of off-by-default features in vcpkg. These features are
+subject to change without notice and should be considered highly unstable.
+
+Non-exhaustive list of off-by-default features:
+
+- `binarycaching`
+
+#### EDITOR
+
+This environment variable can be set to the full path of an executable to be used for `vcpkg edit`. Please see
+`vcpkg help edit` for command-specific help.
+
+#### VCPKG_ROOT
+
+This environment variable can be set to a directory to use as the root of the vcpkg instance. Note that mixing vcpkg
+repo versions and executable versions can cause issues.
+
+#### VCPKG_DEFAULT_VS_PATH
+
+This environment variable can be set to the full path to a Visual Studio instance on the machine. This Visual Studio instance
+will be used if the triplet does not override it via the [`VCPKG_VISUAL_STUDIO_PATH` triplet setting](triplets.md#VCPKG_VISUAL_STUDIO_PATH).
+
+Example: `D:\2017`
+
+#### VCPKG_DEFAULT_TRIPLET
+
+This environment variable can be set to a triplet name which will be used for unqualified triplet references in command lines.
+
+#### VCPKG_FORCE_SYSTEM_BINARIES
+
+This environment variable, if set, suppresses the downloading of CMake and Ninja and forces the use of the system binaries.