doc: Improve contributing documentation

Signed-off-by: Arthur Amstutz <arthur.amstutz@corp.ovh.com>

CONTRIBUTING.md

@@ -1,4 +1,4 @@
-Thanks for wanting to contribute to this project ❤️.
+Thank you for wanting to contribute to this project ❤️
 
 This project accepts contributions. In order to contribute, you should pay attention to a few things:
 
@@ -7,19 +7,98 @@ This project accepts contributions. In order to contribute, you should pay atten
 3. Your code must be tested
 4. GitHub Pull Requests
 
-## Coding and documentation Style:
+The following sections explain in details the contribution guidelines.
+
+# Table of content
+
+- [Submitting Modifications](#submitting-modifications)
+- [Submitting an Issue](#submitting-an-issue)
+- [Coding and Documentation Style](#coding-and-documentation-style)
+- [Adding a New CLI Feature](#adding-a-new-cli-feature)
+    - [1. Service (Business Logic)](#1-service-business-logic)
+    - [2. CLI Command](#2-cli-command)
+    - [3. Create / Edit Command UX Flags](#3-create--edit-command-ux-flags)
+    - [4. Documentation](#4-documentation)
+    - [5. Tests](#5-tests)
+    - [6. Typical Structure Example](#6-typical-structure-example)
+    - [7. Checklist Before Opening a PR](#7-checklist-before-opening-a-pr)
+- [Developer Certificate of Origin (DCO)](#developer-certificate-of-origin-dco)
+
+# Submitting Modifications:
+
+The contributions should be submitted through new GitHub Pull Requests.
+
+# Submiting an Issue:
+
+In addition to contributions, we welcome [bug reports, feature requests and documentation error reports](https://github.com/ovh/ovhcloud-cli/issues/new).
+
+# Coding and documentation Style:
 
 - Code must be formatted with `make fmt` command
 - Name your commands according to the API endpoint
 - If the input body of an API call has more than five parameters or has more than one level of nesting, the corresponding CLI command must have flags '--editor' and '--from-file' to define its parameters.
 
-## Submitting Modifications:
+# Adding a new CLI Feature
 
-The contributions should be submitted through new GitHub Pull Requests.
+This document explains the minimal workflow to add a new feature to the CLI.
 
-## Submiting an Issue:
+## 1. Service (Business Logic)
 
-In addition to contributions, we welcome [bug reports, feature requests and documentation error reports](https://github.com/ovh/ovhcloud-cli/issues/new).
+Place all code that performs HTTP calls and processes responses in a dedicated sub-package under:
+`internal/services/<yourservice>`.
+
+Any logic related to printing or formatting output should be handled by the separate `internal/display` package, which must be used for presenting results or errors to the user. This package handles the formatting depending on the output asked by the user (JSON, YAML, …).
+
+## 2. CLI Command
+
+Add the corresponding command in: `internal/cmd/<yourservice>.go`.
+Follow existing patterns for command registration and use cohesive, single-purpose files.
+
+## 3. Create / Edit Command UX Flags
+
+For resource creation or edition commands, you MUST support:
+
+- `--editor`       open a temporary spec in $EDITOR
+- `--init-file`    create a skeleton file
+- `--from-file`    load full request body from a file
+
+Helper functions already exist to add these flags. See the IAM user creation implementation for reference.
+
+## 4. Documentation
+
+After adding or modifying commands, regenerate docs from the repository root:
+```bash
+make doc
+```
+
+Don't commit changed made to `doc/ovhcloud.md` except if they are manual changes.
+
+## 5. Tests
+
+Add tests for the new command(s) in: `internal/cmd/<yourservice>_test.go` Focus on:
+
+- Flag parsing
+- Error paths
+- Testing the successful execution path by invoking the service layer using mocks
+
+## 6. Typical Structure Example
+
+```
+internal/services/policy/        # HTTP + response handling
+internal/cmd/policy/             # cobra command definitions
+doc/ovhcloud_<generated>.md      # regenerated via make doc
+```
+
+## 7. Checklist Before Opening a PR
+
+- Service code isolated in internal/services
+- Command code added in internal/cmd
+- Supports required create/edit flags (if applicable)
+- Docs regenerated (make doc)
+- Tests added and passing
+- Lint/build succeeds (make build)
+- Keep changes small and consistent with existing patterns
+- Prefer commands that are limited to a single use-case and do not require excessive HTTP calls
 
 # Developer Certificate of Origin (DCO)