Add isMalicious.com cyberthreats api tool

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IsMalicious is a cyberthreat reputation tool that provides real-time threat intelligence for IP addresses and domains.

Added IsMalicious to the README.md file in the Sources section. The entry has been placed alphabetically between "Icewater" and "Infosec - CERT-PA" and includes:

• A link to https://www.ismalicious.com/
• A description highlighting that it's a cyberthreat reputation tool for IP addresses and domains that provides real-time threat intelligence, reputation scores, and historical data