Please provide documentation for --ignore for cli

How to translate:
```
// ignore CSS rules
ignore: {
atrule: ['@font-face'],
rule: [/some-regexp/],
decl: (node, value) => /big-image\.png/.test(value),
},
```
when using cli.

By the way, `--dimensions W1xH1,W2xH2` didn't work for me while stated in #457:
```
Error: ConfigError: "dimensions" must be an array
```
Hopefully: `--dimensions W1xH1 --dimensions W2xH2` did. Good to know we can use this option multiple times.

Best regards,