Add the Sigma Prime audit report in doc/audit/

.gitignore

@@ -5,7 +5,6 @@ analysis-report/
 *.profraw
 *.profdata
 *.prof
-*.pdf
 *.o
 *.s
 *.a

README.md

@@ -78,6 +78,11 @@ Rust) have their own benchmarks. Including benchmarks in the bindings offers a
 more realistic performance estimate, as C-KZG-4844 is not expected to be used
 outside of the bindings.
 
+### Security audit
+
+The source code of c-kzg-4844 was audited by [Sigma Prime](https://sigmaprime.io/) in June 2023. You can find the
+[audit report](https://github.com/ethereum/c-kzg-4844/blob/main/doc/audit/Sigma_Prime_Ethereum_Foundation_KZG_Implementations_Security_Assessment.pdf) in the `doc/audit/` directory.
+
 ### Why C?
 
 The primary reason for choosing C is because