message: Add docs to set VCPKG_ROOT to run tests
diff: @@ -5,6 +5,18 @@ writing new tests and keeping them up to date is also very important. If one's
  code is subtly broken, we'd rather find it out right away than a few weeks down
  the line when someone complains!
  +## Environment Variables
  +
  +You will need to set the environment variable `VCPKG_ROOT` to the location
  +of a copy of the [main vcpkg repo](https://github.com/microsoft/vcpkg) on your
  +machine, in order to run the tests.
  +
  +Failure to do so will result in this error, when some of the tests are run:
  +
  +```
  +Error: Could not detect vcpkg-root.
  +```
  +
  ## Running Tests
  Before anything else, we should know whether you can actually run the tests!
  