docs: add audit report (#259)

Closes #228

The commit hash in the PDF isn't for this repo so I didn't include a
column for the commit hash in the table. IMO we should include one but I
couldn't find the commit hash that was audited so 🤷‍♂️

<!-- This is an auto-generated comment: release notes by coderabbit.ai
-->

## Summary by CodeRabbit

- **Documentation**
- Added an "Audits" section to the README file to include audit
information with columns for Date, Auditor, and Report.

<!-- end of auto-generated comment: release notes by coderabbit.ai -->

README.md

@@ -116,3 +116,9 @@ Markdown files must conform to [GitHub Flavored Markdown](https://github.github.
 
 - [markdownlint](https://github.com/DavidAnson/markdownlint)
 - [Markdown Table Prettifier](https://github.com/darkriszty/MarkdownTablePrettify-VSCodeExt)
+
+## Audits
+
+Date      | Auditor                                       | Report
+----------|-----------------------------------------------|--------------------------------------------------------
+2023/8/18 | [Informal Systems](https://informal.systems/) | [informal-systems.pdf](docs/audit/informal-systems.pdf)