Add PR template and CONTRIBUTING.md

[nsoranzo]

https://docs.github.com/en/github/building-a-strong-community/creating-a-pull-request-template-for-your-repository

CONTRIBUTING.md should reference the licenses for content (CC-BY) and for infrastructure (MIT).

[bedroesb]

@pinarpink I moved the How to contribute page to the root of the directory and named it CONTRIBUTING.md, just like github prefers. It still ends up in the website at the same place

[pinarpink]

@bedroesb ok.

[nsoranzo]

@bedroesb I think CONTRIBUTING.md should be something that can be properly visualised inside GitHub, i.e. straight GitHub-flavoured Markdown. I'd suggest to revert the change and put in CONTRIBUTING.md:

• some details on how to contribute to the website infrastructure, and the licenses as mentioned in this issue;
• a link to the https://rdm.elixir-europe.org/how_to_contribute.html for how to contribute to the website content.

[bedroesb]

I reverted my changes to the original. CONTRIBUTING.md is something for on the to do list.

[nsoranzo]

Thanks!