Add 'Features' section in the README

[darrachequesne]

The kind of change this PR does introduce

• an update to the documentation

Related: #2823

[pherrymason]

👍 Maybe in the part where you explain that Socket.IO is not a WebSocket implementation, you could link to a document where it explains what does that mean?

[darrachequesne]

@raulferras thanks for the feedback! What I meant by that is that since Socket.IO does add some metadata to each packet (i.e. the packet type, the namespace, or the ack id when a message acknowledgement is needed), it will not be able to communicate with someone not talking the same "language". I'll try to rephrase that properly.

[pherrymason]

Thanks to you @darrachequesne, now is much more clear ;)

[darrachequesne]

@raulferras again, thanks for the feedback!