Add a section on HN's demographics

[ayewo]

In a recent comment made by dang on Jan 15, 2021, he gave out general stats on how HN's readership is distributed:

Intellectual curiosity exists everywhere, HN's userbase is 50% outside the US, and at least 90% outside of Silicon Valley. That's important for people to understand.

I think this piece of info, although small, would be a nice addition to repo.