add contributing

CONTRIBUTING.md

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+Contributing
+============
+
+Welcome, so you are thinking about contributing ?
+Awesome, this a great place to start.
+
+Setup
+-----
+
+```bash
+git clone git@github.com:innovativeinnovation/epfl-unit-api.git
+cd epfl-unit-api
+npm i
+```
+
+Test
+----
+
+Unit and integration tests:
+
+```bash
+npm t
+```
+
+Code coverage:
+
+```bash
+npm run coverage
+```
+
+Release
+-------
+
+  1. Bump the correct version (`npm version [<newversion> | major | minor | patch]`)
+  2. Update the file [CHANGELOG.md](CHANGELOG.md)
+  3. Create the tag (`git tag -a v<version> -m "Tagging the v<version> release"`)
+  4. Publish with `npm publish`
+
+License
+-------
+
+Apache License 2.0
+
+(c) William Belle, 2019.
+
+See the [LICENSE](LICENSE) file for more details.