Formalize contribution by creating a CONTRIBUTING.md

[shoriwe]

Is your feature request related to a problem? Please describe.
Right now Odin does not have any document formalizing any way to properly contribute to the language, making harder for new comers to resolve issues without breaking other parts of the project. If there is any document describing this already it should be easily available, we need to think about having Odin's repository as one of the many entry points for new people interested in the language.

Enforcing good practices between contributors, predefining build steps, etc... without the need of reverse engineering the repository structure each time someone new arrives will speedup Odin growth overtime since people will directly tackle new issues

Describe the solution you'd like
Based on the scenario described before, I encourage writing a CONTRIBUTING.md file which could contain at least this headlines:

• Code quality???
• Build steps
• Documentation contribution guidelines
• Core library contribution guidelines
• Vendor libraries contribution guidelines
• Compiler frontend contribution guidelines