add note about distributing bootnode ENRs prior to genesis

specs/phase0/p2p-interface.md

@@ -755,7 +755,7 @@ where the fields of `ENRForkID` are defined as
 * `next_fork_version` is the fork version corresponding to the next planned hard fork at a future epoch. If no future fork is planned, set `next_fork_version = current_fork_version` to signal this fact
 * `next_fork_epoch` is the epoch at which the next fork is planned and the `current_fork_version` will be updated. If no future fork is planned, set `next_fork_epoch = FAR_FUTURE_EPOCH` to signal this fact
 
-*Note*: `fork_digest` is composed of values that are not not known until the genesis block/state are available. Due to this, clients SHOULD NOT form ENRs and begin peer discovery until genesis values are known.
+*Note*: `fork_digest` is composed of values that are not not known until the genesis block/state are available. Due to this, clients SHOULD NOT form ENRs and begin peer discovery until genesis values are known. One notable exception to this rule is the distribution of bootnode ENRs prior to genesis. In this case, bootnode ENRs SHOULD be distributed with `eth2` field set as `ENRForkID(fork_digest=compute_fork_digest(GENESIS_FORK_VERSION, b'\x00'*32), next_fork_version=GENESIS_FORK_VERSION, next_fork_epoch=FAR_FUTURE_EPOCH)`.
 
 Clients SHOULD connect to peers with `fork_digest`, `next_fork_version`, and `next_fork_epoch` that match local values.