message: Mention the Catch2 vendored project in README.
diff: @@ -25,6 +25,7 @@ This project: Licensed under the Apache License, Version 2.0.
  Some directories under `/vendor` are in fact not external libraries vendored-in, but "internally-vendored" - developed as a part of OSVR-Core, and likewise licensed under the Apache License 2.0, but are logically distinct from the Core in their functionality and are thus kept separate in anticipation of potential splitting off into a separate project at some future point. These include `/vendor/comutils` and `/vendor/dummy-sal`.
  - `/cmake` - Git subtree from <https://github.com/rpavlik/cmake-modules> used at compile-time only. Primarily BSL 1.0, some items under the license used by CMake (BSD-style)
  +- `/vendor/catch-single-header-2.8.0` - Header downloaded from <https://github.com/catchorg/Catch2/releases/download/v2.8.0/catch.hpp> - BSL 1.0 license. Used only for building/running tests.
  - `/vendor/com_smart_pointer` - Header (for integrating `boost::intrusive_ptr` with MS COM) extracted from the following project and file: <https://github.com/rengeln/nyx/blob/master/src/Prefix.h>, modified as needed for compatibility. MIT license.
  - `/vendor/discount-windows-bins` - Submodule containing source, build script, and Windows binaries for the "Discount" Markdown processor, used only during the build process on Windows. Upstream at <https://github.com/Orc/discount>. 3-clause BSD license.
  - `/vendor/eigen` - Unpacked release from <http://eigen.tuxfamily.org/> - header-only library under the MPL2 (a file-level copyleft, compatible with proprietary software), define `EIGEN_MPL2_ONLY` to exclude modules with other license from the build (done in OSVR-Core build system).
  