Information for network testing (#4961)

* Documentation for network testing

* Update doc

testing/network_testing/README.md

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+# Lighthouse live network testing
+
+
+## DISCLAIMER
+
+This document describes how to run a lighthouse node with minimal resources and time on a live
+network.
+
+This procedure should ONLY be used for testing networks and never in production and never with
+attached validators. The Lighthouse node described in this state is only a partially functioning
+node.
+
+
+## Overview
+
+We are going to run a single lighthouse node connected to a live network, without syncing and
+without an execution engine. This should only ever be done for testing.
+
+There two main components needed.
+
+1. A lighthouse node that doesn't sync
+2. A fake execution client that does nothing
+
+We will start with the second
+
+## Mock-EL
+
+This is a service that runs and fakes an execution engine. We firstly need to install the lighthouse
+`lcli` tool.
+
+```
+$ make install-lcli
+```
+
+Once installed, run the fake execution client:
+
+```
+$ lcli mock-el --jwt-output-path /tmp/mockel.jwt
+```
+
+This will create a server listening on localhost:8551
+
+## Lighthouse no sync
+
+To create a lighthouse node that doesn't sync we need to compile it with a special flag.
+
+```
+$ cargo build --release --bin lighthouse --features network/disable-backfill
+```
+
+Once built, it can run via checkpoint sync on any network, making sure we point to our mock-el
+
+Prater testnet:
+
+```
+$ lighthouse --network prater bn --execution-jwt /tmp/mockel.jwt --checkpoint-sync-url
+https://prater.checkpoint.sigp.io --execution-endpoint http://localhost:8551
+```
+
+Mainnet:
+
+```
+$ lighthouse --network prater bn --execution-jwt /tmp/mockel.jwt --checkpoint-sync-url
+https://checkpoint.sigp.io --execution-endpoint http://localhost:8551
+```
+
+Additional flags, such as metrics may be added.
+
+
+## Additional Notes
+
+The above is assuming that you have not run the command in the past. If you have a database in
+existence for the network you are testing, checkpoint sync will not start. You may need to add the
+`--purge-db` flag to remove any past database and force checkpoint sync to run.