add rand engine override doc

Author: sbiscigl

docs/OpenSSLRand.md

@@ -0,0 +1,8 @@
+# OpenSSL RAND Engine override and S2N
+
+One of our dependencies [s2n](https://github.com/aws/s2n-tls) overrides the openssl RAND engine, For reasons elaborated on in [their documentation](https://github.com/aws/s2n-tls/blob/main/docs/BUILD.md#rand-engine-override). this functionally means that `RAND_poll` is not implemented on the override and using the function after initializing the SDK will result in error. You can disable this behavior completely by specifying
+
+`-DS2N_OVERRIDE_LIBCRYPTO_RAND_ENGINE=OFF`
+
+as part of the SDK build. This will continue to use the RAND Engine from the detected OpenSSL dependency.
+

docs/README.md

@@ -10,4 +10,5 @@ Here you can find some helpful information on usage of the SDK.
 * [Memory Management](./Memory_Management.md)
 * [Advanced Topics](./Advanced_topics.md)
 * [Coding Standards](./CODING_STANDARDS.md)
-* [Add as CMake external project](./CMake_External_Project.md)
+* [Add as CMake external project](./CMake_External_Project.md)
+* [OpenSSL RAND Engine override and S2N](./OpenSSLRand.md)