README is wrong about the rate limits defined.

The README contains these lines:

```js
const BURST_RATE = 1024 * 1024 * 150; // 150KB/sec burst rate
const FILL_RATE = 1024 * 1024 * 50; // 50KB/sec sustained rate
```

However, these limits as defined are 150 **MiB/sec** and 50 **MiB/sec**, not KB/sec.

Apologies to the server that I definitely did not pound until I realized that this was the case :^)