update the readme to introduce the direct connect mode using virtualcluster

[wonderflow]

In the old way, the cluster-gateway leverages the aggregated APIServer as proxy endpoint for multiple clusters connection. This helps users to discovery clusters easily while provide the security align with K8s RBAC.

But in the recent performance test, we find the same way can be achieved by direct connect the proxy without speaking to the K8s APIServer, which can largely improved the performance.

kubevela/kubevela#5595