update contributing doc

CONTRIBUTING.md

@@ -67,7 +67,11 @@ The configs are consumed by downstream OP Stack software, i.e. `op-geth` and `op
 
 A second module exists in this repo whose purpose is to validate the config exported by the `superchain` module. It is a separate module to avoid import cycles and polluting downstream dependencies with things like `go-ethereum` (which is used in the validation tests).
 
-> Note: when running validation tests locally, you must set Tailscale to use the OP Labs Tailnet exit node in order to be able to connect to CI L1 archive nodes.
+> In order to run validation tests locally you must have access to a L1 archive node. If you are an:
+>
+> - OP Labs core dev: you must set Tailscale to use the Tailnet exit node in order to be able to connect to CI L1 archive nodes
+>
+> - external contributor: you must provide your own L1 archive node and set its RPC endpoint in the `l1.test_rpc` field in the superchain config files (e.g. `superchain/configs/{mainnet,sepolia,sepolia-dev-0}/superchain.yaml`)
 
 ## `ops` Go module