Add a file about contributing.

CONTRIBUTING.md

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+# How to contribute to C3
+
+The C3 project consists of
+
+1. The C3 language itself.
+2. The C3 compiler, called c3c.
+3. The C3 standard library
+4. Various tools, such as the editor plugins
+
+## 1. How to contribute to the C3 language
+
+The C3 language is essentially the language specification. You can contribute to the language by:
+
+1. Filing enhancement requests for changes to the language.
+2. Offering feedback on existing features, on Discord or by filing issues.
+3. Help working on the language specification.
+4. Help working on the grammar.
+
+## 2. How to contribute to the C3 compiler
+
+The C3 compiler consists for the compiler itself + test suites for testing the compiler. 
+You can contribute by:
+
+1. File bugs (by far the most important thing).
+2. Suggest improved diagnostics / error messages.
+3. Refactoring existing code (needs deep understanding of the compiler).
+4. Add support for more architectures.
+5. Add support for more backends.
+
+## 3. How to contribute to the standard library
+
+The standard library is the library itself + test suites for testing the standard library.
+You can contribute by:
+
+1. Filing bugs on the standard library.
+2. Write additional unit tests.
+3. Suggest new functionality by filing an issue.
+4. Work on stdlib additions.
+5. Fix bugs in the stdlib
+6. Maintain a section of the standard library
+
+### How to work on small stdlib additions
+
+If there is just a matter of adding a function or two to an existing module, a pull request
+is sufficient. However, please make sure that:
+
+1. It follows the guidelines for the code to ensure a uniform experience (naming standard, indentation, braces etc).
+2. Add a line in the release notes about the change.
+3. Make sure it has unit tests.
+
+### How to work on non-trivial additions to the stdlib
+
+Regardless whether an addition is approved for inclusion or not, it needs to incubate:
+
+1. First implement it the addition standalone, showing that it’s working well and has a solid design. This has the advantage of people being able to contribute or even create competing implementations
+2. One it is considered finished it can be proposed to be included.
+
+This will greatly help improving the quality of additions.
+
+Note that any new addition needs a full set of unit tests before being included into the standard library.
+
+### Maintain a part of the standard library
+
+A single maintainer is insufficient for a standard library, instead we need one or more maintainer
+for each module. The maintainer(s) will review pull requests and actively work on making the module
+pristine with the highest possible quality.
+
+## 4. How to contribute to various tools
+
+In general, file a pull request. Depending on who maintains it, rules may differ.