Add "How the Cloud SQL Proxy Works" section to README

### Bug Description

The error message says port `3307`, but I have specified port `5432`.

The output from sql proxy
```
Listening on 127.0.0.1:5432
accepted connection from 127.0.0.1:58436
failed to connect to instance: Dial error: failed to dial (connection name = "xxx"): dial tcp x.x.x.x:3307: i/o timeout
```

### Example code (or command)

From my deployment
```
- image: gcr.io/cloud-sql-connectors/cloud-sql-proxy:2.3.0
name: cloud-sql-proxy
args:
- "--private-ip"
- "--structured-logs"
- "--port=5432"
- "--credentials-file=/secrets/credentials.json"
- "xxx"
securityContext:
runAsNonRoot: true
volumeMounts:
- name: cloudsql-instance-credentials
mountPath: /secrets/
readOnly: true
```

### Stacktrace

_No response_

### Steps to reproduce?

1. Deploy it as a sidecar configured with port 5432, pointing at a server it cannot reach.
2. Connect to it.
3. Wait for the timeout
...

### Environment

See the example code for the environment

### Additional Details

_No response_