add note about distributing bootnode ENRs prior to genesis

[djrtwo]

As discussed with @AgeManning and @nisdas, bootnode ENRs need to be distributed prior to learning genesis_validators_root. A note of guidance for how to do this in a standard way has been added.

Note: circleci cache got messed up so bumped version

[djrtwo]

@nisdas and @AgeManning: sanity check that this looks fine (and at least very similar to what you are doing already)

[nisdas]

This looks good to me 👍 , would make sense to handle bootnodes as a special case for this.

[protolambda]

LGTM
In current testnets it's fine, but I wonder if this can be disabled N slots after genesis, or if bootnodes are a special case regardless.

[AgeManning]

Yeah this looks good to me also.
For us, we would likely update the bootnode's ENR after we know the value, as then the bootnode will also be discovered and participate in discovery with other nodes on the network.

[djrtwo]

made a quick note on setting bootnode ENRs to normal values after genesis info is know. thanks!