Update README.md

Author: zguesmi

testchains/nethermind/README.md

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+This folder contains a ready to use docker-compose file that starts:
+* `poco-chain` service. Accessible on http://localhost:8545/
+* `netstats` dashboard to monitor the network status. Accessible on http://localhost:8080.
+* `grafana` to display gas usage and other metrics of the chain. Accessible on http://localhost:8081.
+* `prometheus` and `pushgateway` used to collect metrics from `poco-chain`.
+
+Note: uncomment Docker volumes in compose file to persist data.