Added Immunefi to Audits section

[halkemist]

This pull request adds Immunefi to the list of public security audit sources. Immunefi is a bug bounty platform focused on blockchain and smart contract security.

Checklist

• [x] The URL is not already present in the list (check with CTRL/CMD+F in the raw markdown file).
• [x] Each description starts with an uppercase character and ends with a period.
  Example: solc-js - JavaScript bindings for the compiler.
• [x] Drop all A / An prefixes at the start of the description.
• [x] Avoid using the word Solidity in the description.