Add callout to use the latest release instead of main (#71)

Application developers should use the latest release, instead of main,
to avoid encountering issues with verifying proofs produced by Bonsai,
and other integration issues.

This PR adds a callout to alert users to this

README.md

@@ -1,3 +1,7 @@
+> [!IMPORTANT]  
+> `main` is the development branch for this repository.
+> Application developers should use the [latest release](https://github.com/risc0/risc0-ethereum/releases) instead.
+
 # RISC Zero Ethereum
 
 [RISC Zero] is a zero-knowledge verifiable general computing platform, with [Ethereum] integration.