Update README: clarify the issue, use case and risk

[Sieve](https://doc.dovecot.org/2.3/configuration_manual/sieve/pigeonhole_sieve_interpreter/) script against Spamalot.

## What's that?

This script can be uploaded to your mailserver. Once activated, it filters spam from incoming mail before delivering it into your inbox.

Identified spam mails will be moved into a mailbox named `Junk`. They are not auto-deleted by this script and you can still access and review them.

## Usage

Review the contents of `sieve`, which contains the sieve commands executed on your mail *before uploading*.

Review the contents of `sieve`, which contains the sieve commands executed on your mail, *before uploading*. Please use this sieve script only if you fully understand it. Accept the risks of false positives getting flagged and filtered as spam.

Follow the [Chaosdorf Wiki article](https://wiki.chaosdorf.de/Mail#Details_zum_Mail-Versand), basically you need to install `sieve-connect` and use it to upload the script. Example:

> activate sieve

```

## Contributing

Well appreciated!

If you encounter a suspicious subject or sender, simply add it into the corresponding array in the script. Double-check if it may be prone to false positive matches; you should most likely avoid [very short keywords](https://en.wikipedia.org/wiki/Scunthorpe_problem).

Also, create PRs so others can benefit from your spam as well.