Improve README.md

[thara003]

Improve documentation

Link

https://github.com/ooni/explorer#readme

Describe the problem

In the perspective of a new contributor, the README.md provided seems to be very basic for setting up the project. So it would be good, if we have a CONTRIBUTING.md that covers all aspects of setting up the project locally and contributing.

Describe the improvement

I think we should have CONTRIBUTING.md doc for the project and few things that we can include are:

• pointer to OONI Software Development Guidelines
• any helpful resources for getting started with the project
• specify how to open an Issue or Pull Request using one of our templates
• mention how to start contributing
• direction to reach us, if they have trouble in the setting up of the project

these are things that i have currently in my mind and it can extend to other project oriented things as well.