Documentation: Added CONTRIBUTING.md

CONTRIBUTING.md

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+# Contributing to NoteWrapper
+
+Thank you for your interest in contributing to NoteWrapper.
+
+This project aims to remain minimal, terminal-focused, and easy to extend. Contributions should respect this design philosophy.
+
+---
+
+## Debugging and error handling
+
+Please use the standardized debugging and error-handling functions:
+
+* `debug(...)`
+  Used for formatted debugging output (printf-style formatting).
+
+* `altDebug(...)`
+  A lighter version of `debug()` when full formatting is not required.
+
+* `error(condition, "user" | "program", "error message", ...)`
+  Standard error reporting function:
+
+  * `"user"`: errors caused by user input or configuration
+  * `"program"`: internal or unexpected program errors
+
+---
+
+## Commit naming convention
+
+This is mostly for me, as my commit messages can sometimes be cryptic and hard to follow later.
+
+Format:
+
+```
+<module> (#issue): description
+```
+
+Examples:
+
+* `Editor (#1): add support for Microsoft Word`
+* `README.md: fix typos`
+* `Journal: improve entry parsing logic`
+
+Keep commit messages short and descriptive.
+
+---
+
+## Adding editor support
+
+To add support for a new editor, you must update both documentation and source code:
+
+* Update `README.md`:
+
+  * Add the editor to the **Editor support** table
+  * Update the **Dependencies** section if necessary
+
+* In `src/utils.c`:
+
+  * Add the editor name to `supportedEditor[]`
+  * Increment `numEditor`
+
+* If the editor executable name differs from its logical name (for example `neovim` → `nvim`), update `doesEditorExist()` in `src/utils.c`
+
+* Modify `openEditor()` in `src/utils.c`:
+
+  * If the editor supports a Vivify plugin integration, follow the pattern used for **(neo)vim**
+  * If it does not support integration, NoteWrapper must launch Vivify separately (see implementation used for **nano**)
+
+---
+
+## Before opening a pull request
+
+Before submitting a pull request, ensure that:
+
+* The project compiles successfully using:
+
+  * `make`, or
+  * `nix-build` (on NixOS)
+* There are **no warnings or errors**