docs: add Informal Systems audit PDF (backport #2477) (#2530)

This is an automatic backport of pull request #2477 done by
[Mergify](https://mergify.com).


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Co-authored-by: Rootul P <[email protected]>

README.md

@@ -134,7 +134,7 @@ build
     └── metadata.json
 ```
 
-For the Github release, you just need to upload the `checksums.txt` and `celestia-app_Linux_x86_64.tar.gz` files. 
+For the Github release, you just need to upload the `checksums.txt` and `celestia-app_Linux_x86_64.tar.gz` files.
 
 ### Docs
 
@@ -145,6 +145,10 @@ Package-specific READMEs aim to explain implementation details for developers th
 - [x/blob](./x/blob/README.md)
 - [x/qgb](./x/qgb/README.md)
 
+## Audits
+
+[Informal Systems](https://informal.systems/) audited celestia-app [v1.0.0-rc6](https://github.com/celestiaorg/celestia-app/releases/tag/v1.0.0-rc6) in Q3 of 2023. See [audit/informal-systems.pdf](audit/informal-systems.pdf) for the full report.
+
 ## Careers
 
 We are hiring Go engineers! Join us in building the future of blockchain scaling and interoperability. [Apply here](https://jobs.lever.co/celestia).