Update contributing.md link (#804)
Update contributing.md link (#804)

When I click the link, it returns 404 because it looks like the contributing.md was in the main folder (and, by the way, was uppercase and now it is not) and now is in the docs/ folder.
mwouts/jupytext,mwouts/jupytext,mwouts/jupytext,mwouts/jupytext,mwouts/jupytext,mwouts/jupytext,mwouts/jupytext,mwouts/jupytext,mwouts/jupytext,mwouts/jupytext