Add Squiggle to "Who is using" list

[gosquiggle]

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 squiggle.io to the "Who is using simple-peer" list.

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

n/a

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

n/a

[jzombie]

This looks interesting. Saw this while watching my three-year-old daughter and her mom painting together and think it could be something she would like.

[gosquiggle]

Well, thank you @feross for making it, err, simple!