adding CONTRIBUTING.md template

[tminussi]

I have created a CONTRIBUTING.md template which you can use.

Since I don't know exactly what you want, I've set placeholders on most places. If you want, just leave a message with the text you want and I will replace them accordingly.

With the template in place though it will be easier to figure out what is relevant to this project or not.

[i5o]

Hey @tminussi , thanks for this contribution.
However we are closing it temporaly since it has too many placeholders, we will re-open in the future when we have everything ready to fill them :).

Thanks again