Provide contribution documentation

Provide documentation on how to contribute snippets

It should explain how to :
- Add a new snippet
- How it should be documented
- What the Named Values naming convention is

No file or documentation entry point is named in the issue. Start by reviewing the repository structure and existing snippet documentation, then identify where contributor guidance belongs. Done means the guidance explains adding a snippet, documenting it, and following the Named Values naming convention.

Written by the indexing model from the issue text.