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doc/CONTRIBUTING.md

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+# Contributing
+## Overview
+`rts` strives to run on a wide variety of platforms, including low-end
+and old hardware, such as PSX or even `i486` computers. This is possible
+thanks to the use of portable source code and the wide availability of
+`sdl-1.2`. However, `rts` should be easy to extend to other platforms.
+
+## License
+As stated on [the project license](../LICENSE), `rts` is licensed by
+the GNU General Public License, either version 3 of the license, or
+later. Anyone contributing their own source code to this project must
+adhere to this license. Also, only source code from other projects
+compatible with the project license is allowed.
+
+## C language revision
+Because of C11 not being available for the `i386-mingw32` toolchain, C99
+is the selected language revision for this project. On the other hand,
+feel free to use any helpful C99 features.
+
+## Compiler extensions
+Avoid compiler-specific extensions unless absolutely needed. While this
+project is currently built using the GNU toolchain, those using other
+compilers might appreciate not having to deal with such extensions.
+
+## Platform-specific code
+Separate platform-agnostic code from platform-specific code. The usual
+method used by this project is to separate platform-specific code into
+its own directory. Please, do not use macros for this purpose, since
+they make code more difficult to read and maintain.
+
+This is the typical tree structure of a module:
+
+```
+module_name/
+├── CMakeLists.txt
+├── inc
+│   └── file.h
+├── privinc
+│   └── file.h
+├── ps1
+│   ├── inc
+│   │   └── module_name
+│   │       └── file.h
+│   ├── privinc
+│   │   └── ps1
+│   │       └── file.h
+│   └── src
+│       └── file.c
+└── sdl-1.2
+    ├── inc
+    │   └── module_name
+    │       └── file.h
+    ├── privinc
+    │   └── sdl-1.2
+    │       └── file.h
+    └── src
+        └── file.c
+```
+
+Where:
+
+- `src` holds the source files defining the implementation of a given module.
+- `inc` holds public header files accessible to other modules.
+- `privinc` holds public header files accessible to other modules.
+- `ps1` and `sdl-1.2` are two of the platforms supported by this project.
+
+## C code conventions
+
+- Use Allman style for brackets.
+- Strive for maximum `const`-correctness.
+- Keep a hard 80-column limit.