Update README to add section detailing people using libp2p

[achingbrain]

A section should be added to the readme detailing users building useful and innovative tools on top of js-libp2p.

Some projects to include are:

• https://github.com/ChainSafe/lodestar
• https://github.com/hoprnet
• https://github.com/ipfs/js-ipfs

And others.