Our contributing doc update

[rkooo567]

For open source programs, clear contribution guide is essential. The current contributing doc is a little bit deprecated and does not contain enough information to start contributing to the project. I will modify the contributing doc to include

1. CI Process (Especially how travis and jenkins work together)
2. formatter is changed from shell script to https://github.com/ambv/black
3. How to run integration test locally
4. How to reproduce CI environment
5. How to add submodules to the clipper_admin.

Please let me know if you can come up with any other things to add.

[rkooo567]

We also need

tutorial

1. Kubernetes deployment (ideally with minikube)
2. How to debug (using docker logs, fluentd, and etc.)

How to start contribute

Especially, we can expose some tasks to public