Create CONTRIBUTING.md

Author: recp

CONTRIBUTING.md

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+# CONTRIBUTING
+
+Any contributions (code, documentation, ...) are welcome. This project uses [cmocka](http://cmocka.org) for testing, you may need to check their documentation 
+
+# New Features
+- This library may not accept all new features, it is better to create an issue and get approval before coding
+- You must add test for every new feature
+- The feature must be compiled in both UNIX/POSIX systems (e.g. macos, linux...) and Windows
+
+# Code Style
+This library is written with C99, don't try to add C++ files (yes it can compiled into lib),
+if you have enough reason to add C++ files than create an issue and get approval before coding,
+
+- All functions must have `glm` prefix
+- Lines should be wrapped at 80 characters.
+- Don't invent new style for existing ones
+- Use C89 style comments (`/* comments */`) not C++ style comments (`// comments`)
+- Don't use TABs instead use 2 spaces for TABs
+- All indents must be 2 spaces, not 1 nor 4 space
+- All functions in `include` folder must be exported by `CGLM_EXPORT` and wrapped by `extern "C" {` for C++
+- Crate new line for return type, attribs:
+
+```C
+CGLM_INLINE
+void
+glm_mul(mat4 m1, mat4 m2, mat4 dest)
+```
+
+not acceptable:
+
+```C
+CGLM_INLINE void glm_mul(mat4 m1, mat4 m2, mat4 dest)
+```
+- Variables must be declared at the top of a scope before usage:
+```C
+int x;
+int y;
+
+x = y = 0;
+```
+
+not acceptable:
+
+```C
+int x;
+
+x = 0;
+int y = 0;
+```
+
+- All files must retain same LICENSE statement
+- Code with warnings will not be accepted, please suppress them (not by disabling them)
+- Run code anaylysis before submitting pull requests, if you use Xcode you can enable Sanitizer in scheme, you can use valgrind in linux