updated contributing guide + PR template

[egpbos]

This PR introduces some changes to the Guide's contribution guide. Open to feedback!

• The scope has been made a more explicit, not only listing what should go in, but also what shouldn't.
• We stole the Blog's idea of having an Editorial Board.
• The pull request template has been made more explicit to make sure that key points of the guidelines are not forgotten.

[lyashevska]

Looks good. Nice comments by others.

[egpbos]

@bouweandela @c-martinez I processed all review comments. I think this is good to be merged. Can one of you approve?