Added CONTRIBUTING.md [skip ci]

CONTRIBUTING.md

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+# Contributing to toml++
+Contributions are very welcome! Either by [reporting issues] or submitting pull requests.
+If you wish to submit a PR, please be aware that:
+- The single-header file `toml.hpp` is generated by a script; make your changes in the files in
+    `include`, **not** in `toml.hpp`.
+- Your changes should compile warning-free on at least one of gcc 8.3.0, clang 8.0, and MSVC 19.2X
+    (Visual Studio 2019). All three is a bonus.
+- You should regenerate the single-header file as part of your PR (a CI check will fail if you don't).
+
+### Regenerating toml.hpp
+1. Make your changes as necessary
+2. If you've added a new header file that isn't going to be transitively included by one of the
+    others, add an include directive to `include/toml++/toml.h`
+3. Run `python/generate_single_header.py`
+
+### Building and testing
+Testing is done using [Catch2], included in the respository as a submodule under `extern/Catch2`.
+The first time you want to begin testing you'll need to ensure submodules have been fetched:  
+```bash
+git submodule update --init --recursive extern/Catch2
+```
+
+#### Windows
+
+Install [Visual Studio 2019] and [Test Adapter for Catch2], then open `vs/toml++.sln` and build the
+projects in the `tests` solution folder. Visual Studio's Test Explorer should pick these up and
+allow you to run the tests directly.
+
+If test discovery fails you can usually fix it by clicking enabling
+`Auto Detect runsettings Files` (settings gear icon > `Configure Run Settings`).
+
+#### Linux
+Install [meson] and [ninja] if necessary, then test with both gcc and clang:
+```bash
+CXX=g++ meson build-gcc
+CXX=clang++ meson build-clang
+cd build-gcc && ninja && ninja test
+cd ../build-clang && ninja && ninja test
+```
+
+[Visual Studio 2019]: https://visualstudio.microsoft.com/vs/
+[Test Adapter for Catch2]: https://marketplace.visualstudio.com/items?itemName=JohnnyHendriks.ext01
+[reporting issues]: https://github.com/marzer/tomlplusplus/issues
+[Catch2]: https://github.com/catchorg/Catch2
+[meson]: https://mesonbuild.com/Getting-meson.html
+[ninja]: https://github.com/ninja-build/ninja/wiki/Pre-built-Ninja-packages