Axios error code introduction

### Section/Content To Improve

https://github.com/axios/axios/blob/v1.x/README.md#handling-errors

### Suggested Improvement

I notice you have refactored the AxiosError type and add some error code declaration, but I cannot find any documents to introduce these error codes. Is it possible to add some content to introduce the error code?

### Relevant File(s)

```bash
README.md
```
