[enh] #104 add CONTRIBUTE.md file with release workflow

CONTRIBUTE.md

+# Contribute guide
+
+## Release workflow
+
+To handle a release, you have to respect this workflow:
+
+* Verify all features and bug fixes are merged in the `dev` branch.
+* Checkout on the `dev` branch
+* Update the `CHANGELOG.md` file and commit
+* Run the `release.sh` script with the version semantic number as argument:
+
+```bash
+./release.sh 0.50.0
+```
+
+* A new commit is added with the version number and a tag in git.
+* Merge all new commits from `dev` to `master` on GitLab with a merge request.
+* Release on PyPI from the GitLab pipeline manual job of the `master` branch.