Set up contributing guidelines

[augusto-herrmann]

Having a document with contributing guidelines helps any project gather meaningful and useful contributions, and that's why it's encouraged by Github. They also give instructions on how to set it up.

We already have some of that in the README.md:

• information on how to contribute new data, and
• instructions on how to set up a development environment for the website.

A new CONTRIBUTING.md file should have that and also include:

• instructions for opening new issues and pull requests
• explicit csv dialect and schema orientation
• frictionless data package validation for new data submissions
• ... anything else?