update contributing instruction set

.github/CONTRIBUTING.md

@@ -8,3 +8,18 @@ Those are the main contributing guidelines for contributing to this project:
 - Use [Pull Request](https://help.github.com/en/github/collaborating-with-issues-and-pull-requests/about-pull-requests) mechanism and please be patient while waiting for reviews.
 - Remain polite and civil in all exchanges with the maintainers and other contributors.
 - Any issue submitted which does not respect provided template, or lack of information, will be considered as invalid and automatically closed.
+
+## Get started
+
+In order to contribute, the safest is to create your [own fork of spleeter](https://help.github.com/en/github/getting-started-with-github/fork-a-repo) first. The following set of commands will clone this new repository, create a virtual environment provisioned with the dependencies and run the tests (will take a few minutes):
+
+```bash
+git clone https://github.com/<your_name>/spleeter
+python -m venv spleeterenv && source spleeterenv/bin/activate
+pip install -r requirements.txt && pip install pytest pytest-xdist
+make test
+```
+
+You can then make your changes and experiment freely. Once you're done, remember to check that the tests still run. If you've added a new feature, add tests!
+
+Then finally, you're more than welcome to create a [Pull Request](https://help.github.com/en/github/collaborating-with-issues-and-pull-requests/creating-a-pull-request-from-a-fork) in **Spleeter** main repo. We will look at it as soon as possible and eventually integrate your changes in the project.