Clarify drop-frequency usage

In the README it's unclear how to properly utilize drop-frequency. For example it's listed as

> drop-frequency how often should bytes be dropped? (byte count % drop frequency).

However how does one interpret "byte count % drop frequency"? Does

```
crapify start --port=5000 --speed=3000 --concurrency=2 --drop-frequency=4 % 20
```

mean that since frequency is a number of some repeating thing per time unit does the above translate to drop 4 bytes every 20 seconds, or drop 4% of bytes per 20 milliseconds, any conceivable combination therein, or something else entirely?

Furthermore it might help to also include drop-frequency in the Usage section of the README as well such as:

```
>crapify start --port=5000 --speed=3000 --concurrency=2 --drop-frequency=4 % 20
```