Add note about Go support to README (#5851)

Author: leighmcculloch

README.md

@@ -60,4 +60,8 @@ As of tag [**stellar-go-2025-10-29_10-56-50**](https://github.com/stellar/go/rel
 
 Contributions are welcome! See [CONTRIBUTING.md](CONTRIBUTING.md) for more details.
 
+## Go Versions Supported
+
+The packages in this repository are tested against the two most recent major versions of Go, because only [the two most recent major versions of Go receive security updates](https://go.dev/doc/security/policy).
+
 [Stellar Development Foundation]: https://stellar.org