Missing English document for 《Add Hysteria2 Protocol》

[xiaokangwang]

Missing document for: v2fly/v2ray-core#2721

@JimmyHuang454 would you like to work on writing its documentation?

[xiaokangwang]

Done with a7b5aa1

[arinc9]

The English documentation for it is still missing.

[JimmyHuang454]

I will update it ASAP.

[arinc9]

@JimmyHuang454 Could you apply #474 first, and then add the English documentation on top of it?