Add Jam to README.md

[mitschabaude]

What is the purpose of this pull request? (put an "X" next to item)

[X] Documentation update [ ] Bug fix [ ] New feature [ ] Other, please explain:

What changes did you make? (Give an overview)

I added Jam to the list of Apps using simple-peer. Jam is an open source Clubhouse alternative. Source code is on Gitlab.

Which issue (if any) does this pull request address?

Is there anything you'd like reviewers to focus on?