docs: add contributing page (#567)

* prettier: force proseWrap rule

* contributing: create

.prettierrc

+{
+  "endOfLine": "lf",
+  "printWidth": 80,
+  "proseWrap": "always"
+}

CONTRIBUTING.md

+# Contributing
+
+We would love for you to contribute to the backoffice of [Code du travail
+numérique][link-cdtn] and help make it even better than it is today! As a
+contributor, here are the guidelines we would like you to follow:
+
+- [Commit Message Guidelines](#commit-message-guidelines)
+
+## Commit Message Guidelines
+
+Each commit message consists of a **type**, a **scope** and a **subject**:
+
+```
+<type>(<scope>): <subject>
+```
+
+The **type** is mandatory and the **scope** of the header is optional.
+
+Any line of the commit message cannot be longer 100 characters! This allows the
+message to be easier to read on GitHub as well as in various git tools.
+
+**Examples:**
+
+- `docs(changelog): update changelog to 1.12.5`
+- `fix(release): need to depend on latest rxjs and zone.js`
+- `ci: configure dependabot`
+
+Do not hesitate check [existing commits][link-cdtb-commits] to get a better
+understanding.
+
+### Revert
+
+If the commit reverts a previous commit, it should begin with `revert:`,
+followed by the header of the reverted commit. In the body it should say:
+`This reverts commit <hash>.`, where the hash is the SHA of the commit being
+reverted.
+
+### Type
+
+Must be one of the following:
+
+- **build**: Changes that affect the build system or external dependencies.
+- **chore**: Changes to our CI configuration files and scripts.
+- **ci**: Changes to our CI configuration files and scripts.
+- **docs**: Documentation only changes.
+- **feat**: A new feature.
+- **fix**: A bug fix.
+- **perf**: A code change that improves performance.
+- **refactor**: A code change that neither fixes a bug nor adds a feature.
+- **style**: Changes that do not affect the meaning of the code.
+- **test**: Adding missing tests or correcting existing tests.
+
+### Scope
+
+The scope should be the name of the npm package affected (as perceived by the
+person reading the changelog generated from commit messages.
+
+The following is the list of supported scopes:
+
+- **api**
+- **admin**
+- **contrib**
+
+There are currently a few exceptions to the "use package name" rule:
+
+- **packaging**: Used for changes that change the npm package layout in all of
+  our packages, e.g. public path changes, package.json changes done to all
+  packages, d.ts file/format changes, changes to bundles, etc.
+- **changelog**: Used for updating the release notes in CHANGELOG.md
+- none/empty string: useful for `style`, `test` and `refactor` changes that are
+  done across all packages (e.g. `style: add missing semicolons`) and for docs
+  changes that are not related to a specific package (e.g.
+  `docs: fix typo in tutorial`).
+
+### Subject
+
+The subject contains a succinct description of the change:
+
+- Use the imperative, present tense: "change" not "changed" nor "changes".
+- Don't capitalize the first letter.
+- No dot (.) at the end.
+
+---
+
+[link-cdtb-commits]:
+  https://github.com/SocialGouv/code-du-travail-backoffice/commits/master
+[link-cdtn]: https://codedutravail.num.social.gouv.fr