Add a "Where to buy BTS" section?

[abitmore]

We had pull requests which wanted to add centralized exchanges, E.G.

• Update README.md - added MXC Exchange #38
• Update README.md #12

Although people can buy or sell BTS via the bridges, they can do so via centralized exchanges too. A difference is, when a bridge is used, the user interact with the chain, but when a centralized exchange is used, there may be no activity on the chain.

[shulthz]

I think we should take off bts from all the centralized exchanges as the users of these markets didn't interact with the chain.

It seemed we spent much money on MXC, we should claim the money back, is it right, abit?

I think Tether shouldn't publish USDT on the other chains, that's very stupid.