docs: document OPENSHELL_SSH_HANDSHAKE_SECRET in Getting Started (#1287)

The Podman and Kubernetes compute drivers require
OPENSHELL_SSH_HANDSHAKE_SECRET to be set. This was introduced in
2e0afea ("feat(vm): derive guest rootfs from sandbox images (#957)"),
which exempted only the Docker and VM drivers from the check.

The Getting Started instructions in CONTRIBUTING.md didn't mention
the variable, so developers using Podman (the default on systems
where it is installed) hit an opaque configuration error on first run.

Add the export as a separate setup step with a comment explaining
which drivers require it.

Signed-off-by: Russell Bryant <russell.bryant@gmail.com>

Author: russellb

CONTRIBUTING.md

@@ -152,6 +152,10 @@ cargo build -p openshell-prover --features bundled-z3
 # One-time trust
 mise trust
 
+# Podman and Kubernetes drivers require an SSH handshake secret.
+# Set any value for local development:
+export OPENSHELL_SSH_HANDSHAKE_SECRET=dev-secret
+
 # Run a standalone gateway for local development
 mise run gateway
 ```