update documentation for victorops support

[alonm-totango]

Hi, I can't find the exact doc file I see on the official website but looks like your configuration for Victorops need to update. there are missing fields that are very useful like custom_fields.

you can see the options here: https://help.victorops.com/knowledge-base/victorops-prometheus-integration/ and https://help.victorops.com/knowledge-base/incident-fields-glossary/

[hdost]

I think this need to be moved to prometheus/alertmanager