For disk-space-saving erigon configuration, outline pruning CLI options in readme

[TimDaub]

• Erigon can be used such that state before a certain block isn't stored on disk and is pruned
• An exemplary configuration can be ./build/bin/erigon --datadir=/mnt/erigon --prune=htcr --prune.h.before=10000000 --prune.t.before=10000000 --prune.c.before=10000000 where certain state isn't stored before block 10000000 and so a lot of state space is saved.
• Outline this configuration in the readme to make neume-network accessible to more users