add a *how to contribute* section

[KevCaz]

This should include all the information to contribute efficiently to the post. For instance, it will cover:

1. how to proceed for new contributors as @david-beauchesne explains in:

Ok, so we need to update the authors/contributors for the site. It is quite simple, you need to fill in a separate yaml file per author. Go to ./data/authors/, copy a file there and fill out the yaml header with your information. Push and commit your newly created yaml file with your name as the file name. Keep the name super(!!) simple, it's what we use in the posts to identify authors. The website will be automatically updated once you add in your information.

2. how we should proceed, direct push on the dev branch or a PR process? (I'd suggest the latter);

3. how files should be named;

4. etc.

[KevCaz]

I am adding a CONTRIBUTING files in the repo. Please comment and edit. If you feel something important is missing, please open a separate issue.