docs: add security policy (#1495)

SECURITY.md

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+# Security
+
+For more information about Aragon-related security and contact information, please visit our [security policy page](https://wiki.aragon.org/association/security/).
+
+## Status
+
+The smart contracts underlying Aragon organizations have passed a number of [independent professional security audits](https://wiki.aragon.org/association/security/). Ongoing, rolling reviews are also conducted from time to time as new changes and releases are frozen.
+
+There is an ongoing [bug bounty program](https://wiki.aragon.org/dev/bug_bounty/) with a bounty pool of $250,000 USD for rewarding hackers who find new security vulnerabilities. [More information](https://wiki.aragon.org/dev/bug_bounty/).