Create CONTRIBUTING.md

CONTRIBUTING.md

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+# Contributing to Xash3D FWGS
+
+Third-party patches for Xash3D FWGS are great idea. 
+But if you want your patch to be accepted, we can give you few guidelines.
+
+## If you are reporting bugs
+
+1. Check you are using latest version. You can build latest Xash3D FWGS for yourself, look to README.md.
+2. Check open issues is your bug is already reported and closed issues if it reported and fixed. Don't send bug if it's already reported.
+3. Now you can write bugreport.
+4. Describe steps to reproduce bug.
+5. Describe which OS and architecture you are using.
+6. Re-run engine with `-dev 2 -log` arguments, reproduce bug and post engine.log which can be found in your working directory.
+7. Attach screenshot if it will help clarify the situation.
+
+## If you are contributing code
+
+### Which branch?
+
+* We recommend using `master` branch. But you still can use any of our branches, if they are up-to-date.
+
+### Third-party libraries
+
+* Philosophy of any Xash Project by Uncle Mike: don't be bloated. We follow it too.
+* There is allowed only these libraries, which is used in Linux GoldSrc version or if there is a REAL reason to use library.
+
+### Portability level
+
+* Xash3D have it's own crt library. It's recommended to use it. It most cases it's just a wrappers around standart C library.
+* If your feature need platform-specific code, try to implement to every supported OS and every supported compiler.
+* You must put it under appopriate macro. It's a rule: Xash3D FWGS must at least compile everywhere.
+
+| OS | Macro |
+| -- | ----- |
+| Linux | `defined(__linux__)` |
+| FreeBSD | `defined(__FreeBSD__)` |
+| NetBSD | `defined(__NetBSD__)` |
+| OpenBSD | `defined(__OpenBSD__)` |
+| OS X/iOS | `defined(__APPLE__)` and TargetConditionals macros |
+| Windows | `defined(_WIN32)` |
+| Android | `defined(__ANDROID__)` |
+| Emscripten | `defined(__EMSCRIPTEN__)` |
+
+### Code style
+
+* This project uses mixed Quake's and HLSDK's C/C++ code style convention. 
+* In short:
+  * Use spaces in parenthesis. Even in empty.
+  * Use tabs, not spaces, for indentation.
+  * Any brace must have it's own line.
+  * Short blocks, if statements and loops on single line are allowed.
+  * Try to avoid magic numbers.
+* If you unsure, try to mimic code style from anywhere else of engine source code.