Deprecating WebAudio demo-list

We have https://github.com/WebAudio/demo-list which says you can submit
demos by sending email here or doing a pull request.

Can we just stop saying that and tell them to post their demos somewhere
else?

No one is looking at pull requests and no one is going to review the actual
demo, so let's just make it clear that we're not accepting any new entries.

Thoughts?