Add CONTRIBUTING.

CONTRIBUTING.md

+Contributing to miniaudio
+=========================
+
+Found a Bug?
+------------
+If you've found a bug you can create a bug report [here on GitHub](https://github.com/dr-soft/miniaudio/issues).
+The more information you can provide, the quicker I'll be able to get it fixed. Sample programs and files help a
+lot, as does a detailed list of steps I can follow to reproduce the problem.
+
+You can also submit a pull request which, provided your fix is correct and well written, is the quickest way to
+get the bug fixed. See the next section for guidance on pull requests.
+
+
+Pull Requests
+-------------
+If you want to do actual development on miniaudio, pull requests are the best place to start. Just don't do any
+significant work without talking to me first. If I don't like it, it won't be merged. Always base your pull
+request branch on the "dev" branch. The master branch contains the latest release, which means your pull request
+may not be including the lastest in-development changes which may result in unnecessary conflicts.
+
+I need to review your pull requests before merging. If your pull request is non-trivial, try to break it up into
+logical bite sized commits to make it easier for review, but make sure every commit compiles.
+
+When it comes to coding style I'm fairly relaxed, but be a professional and respect the existing coding style,
+regardless of whether or not you like it. It's no big deal if something slips, but try to keep it in mind. Some
+things in particular:
+  * C89. `/*...*/` style comments and variables declared at the top of the code block are the main thing.
+  * Spaces instead of tabs. 4 spaces per tab.
+  * Don't add a third party dependency. If you do this I'll immediately reject your pull request.
+
+I'm not going to outline specific coding styles - just look at the existing code and use common sense.
+
+
+Licensing and Credits
+---------------------
+miniaudio is dual licensed as a choice of public domain or MIT-0 (No Attribution), so you need to agree to release
+your contributions as such. I also do not maintain a credit/contributions list. If you don't like this you should
+not contribute to this project.
+
+
+Before You Ask:
+---------------
+* No, I'm not switching away from C89.
+* No, I'm not adding a third license option.
+* No, I'm not adding you to any kind of credit list.
+* No, I'm splitting the project out into multiple files.
+* No, I'm not adding support for CMake nor any other kind of build system.
+* No, there's no Code of Conduct and I'm not adding one. Just don't be unpleasant.
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