Add section in CONTRIBUTING.md about cloud 9 setup

[miketaylr]

@dshgna got the app running on Cloud 9 (https://c9.io/), with the following:

app.run(host=os.getenv(‘IP’, ‘0.0.0.0’),port=int(os.getenv(‘PORT’, 8080)))

Would be good to add that somewhere to the env setup for other people who might be using that.

[miketaylr]

This is fixed now, thanks @dshgna!

(One cool trick to do in a future PR is to set the Pull Request title to have the title be "Fixes #934: blah blah blah" that way when it gets merged it will automatically close the issue).