Add Exoo to README.md

[NathanPar]

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) Add a project (exoo.io) to README.md

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

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