What do you mean with "contributions"?
We have a
Wiki for such additions to SFML, one can post their stuff there and if one wants to let the forum know, there's a
wiki subforum, where one could write a post.
if it's just a project, we have the
project subforum.
If you meant a simple code contribution to SFML itself, then you can always have a discussion here and the source should be made available through a pull request.
