add governance docs

CHANGELOG.md

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+# Plutus Apps Changelog
+
+All major changes to this project will be documented in this file.

CODEOWNERS.md

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+# Plutus Apps Codeowners
+
+# These owners will be the default owners for everything in the repository.
+* @
+
+# General reviewers per PR
+#                        Name   Name          
+*                        @      @  

SECURITY.md

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+# Security Policy
+
+## Reporting a Vulnerability
+
+Please report (suspected) security vulnerabilities to security@intersectmbo.org. You will receive a
+response from us within 48 hours. If the issue is confirmed, we will release a patch as soon
+as possible.
+
+Please provide a clear and concise description of the vulnerability, including:
+
+* the affected version(s) of plutus-apps,
+* steps that can be followed to exercise the vulnerability,
+* any workarounds or mitigations
+
+If you have developed any code or utilities that can help demonstrate the suspected
+vulnerability, please mention them in your email but ***DO NOT*** attempt to include them as
+attachments as this may cause your Email to be blocked by spam filters.
+See the security file in the [Cardano engineering handbook](https://github.com/input-output-hk/cardano-engineering-handbook/blob/main/SECURITY.md).