Include rate limiting documentation

## What is the problem your feature solves, or the need it fulfills?

I am interested to use Pingora but without digging through source code I am unsure if it can rate limit various metrics as I hope. I want to enforce limits such as:

- RPS per downstream connection
- RPS per upstream host (basically a sum of downstream RPS)
- Total downstream connections for a given upstream host
- Total downstream connections for an IP

## Describe the solution you'd like

I would like to know when and what I can rate limit. I've read the life of a request so understand there are several points I can hook into. I was just hoping the pingora-limits crate would help me out and have some examples or documentation without having to read the source.

## Describe alternatives you've considered

Reading source code.