Added Code4rena to Audits section

[halkemist]

This pull request adds Code4rena to the list of public security audit sources. Code4rena provides a competitive platform where auditors compete to identify and eliminate high-severity bugs before production.

Checklist

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

[bkrem]

Both commits on this branch (Code4rena + Immunefi) have already landed via #135 — that PR's branch happened to contain both, so a rebase + squash-merge of #135 brought Code4rena in too. Closing as superseded. Thanks for the contribution!