[META] Add a CONTRIBUTING.md (#674)

CONTRIBUTING.md

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+# Winit Contributing Guidelines
+
+## Scope
+
+Winit aims to provide a generic platform abstracting the main graphic platforms (Windows, macOS, X11,
+Wayland, Android, iOS and the web platform via Emscripten).
+
+Most platforms expose capabilities that cannot be meaningfully transposed to the others. Winit does not
+aim to support every single functionality of every platform, but rather to abstract the set of
+capabilities that is common to all platforms. In this context, APIs exposed in winit can be split into
+different "support levels":
+
+- Tier 1: features which are in the main scope of winit. They are part of the common API of winit, and
+  are taken care of by the maintainers. Any part of these features that is not working correctly is
+  considered a bug in winit.
+- Tier 2: some platform-specific features can be sufficiently fundamental to the platform that winit can
+  integrate support for them in the platform-specific part of the API. These features are not considered
+  directly handled by the maintainers of winit. If you have a strong incentive to have such a feature
+  integrated in winit, consider implementing it and proposing yourself to maintain it in the future.
+- Tier 3: these features are not directly exposed by winit, but rather can be implemented using the
+  raw handles to the underlying platform that winit exposes. If your feature of interest is rather
+  niche, this is probably where it belongs.
+
+The exact list of supported Tier 1 features is tracked in this issue:
+[#252](https://github.com/tomaka/winit/issues/252).
+
+## Reporting an issue
+
+When reporting an issue, in order to help the maintainers understand what the problem is, please make 
+your description of the issue as detailed as possible:
+
+- if it is a bug, please provide clear explanation of what happens, what should happen, and how to
+  reproduce the issue, ideally by providing a minimal program exhibiting the problem
+- if it is a feature request, please provide a clear argumentation about why you believe this feature
+  should be supported by winit
+
+## Making a pull request
+
+When making a code contribution to winit, before opening your pull request, please make sure that:
+
+- you tested your modifications on all the platforms impacted, or if not possible detail which platforms
+  were not tested, and what should be tested, so that a maintainer or another contributor can test them
+- you updated any relevant documentation in winit
+- you left comments in your code explaining any part that is not straightforward, so that the 
+  maintainers and future contributors don't have to try to guess what your code is supposed to do
+- your PR adds an entry to the changelog file if the introduced change is relevant to winit users
+
+Once your PR is open, you can ask for review by a maintainer of your platform. Winit's merging policy
+is that a PR must be approved by at least two maintainers of winit before being merged, including
+at least a maintainer of the platform (a maintainer making a PR themselves counts as approving it).
+
+## Maintainers & Testers
+
+Winit is managed by several people, each with their specialities, and each maintaining a subset of the
+backends of winit. As such, depending on your platform of interest, your contacts will be different.
+
+This table summarizes who can be contacted in which case, with the following legend:
+
+- `M`: is a main maintainer for this platform
+- `R`: can review code for this platform
+- `T`: has the ability of testing the platform
+- ` `: knows nothing of this platform
+
+| Platform | Windows | macOS | X11 | Wayland | Android | iOS | Emscripten |
+| :--- | :---: | :---: | :---: | :---: | :---: | :---: | :---: |
+| @francesca64 | R | (M) | M |  | (M) | (M) | |
+| @mitchmindtree | T |  | T | T |  |  |  |
+| @Osspial | M |  | T | T | T |  | T |
+| @vberger |  |  | T | M |  |  |  |
+
+Platforms macOS, Android and iOS are currently being handled by @francesca64, but we are looking for
+maintainers for these!