add random subnet subscriptions for phase 0 stability

specs/validator/0_beacon-chain-validator.md

@@ -55,6 +55,7 @@
                 - [Aggregate signature](#aggregate-signature-1)
             - [Broadcast aggregate](#broadcast-aggregate)
                 - [`AggregateAndProof`](#aggregateandproof)
+    - [Phase 0 attestation subnet stability](#phase-0-attestation-subnet-stability)
     - [How to avoid slashing](#how-to-avoid-slashing)
         - [Proposer slashing](#proposer-slashing)
         - [Attester slashing](#attester-slashing)
@@ -79,6 +80,8 @@ All terminology, constants, functions, and protocol mechanics defined in the [Ph
 | - | - | :-: | :-: |
 | `ETH1_FOLLOW_DISTANCE` | `2**10` (= 1,024) | blocks | ~4 hours |
 | `TARGET_AGGREGATORS_PER_COMMITTEE` | `2**4` (= 16) | validators | |
+| `RANDOM_SUBNETS_PER_VALIDATOR` | `2**0` (= 1) | subnets | |
+| `EPOCHS_PER_RANDOM_SUBNET_SUBSCRIPTION` | `2**8` (= 256) | epochs | ~27 hours |
 
 ## Becoming a validator
 
@@ -418,6 +421,10 @@ Where
 * `selection_proof` is the signature of the slot (`slot_signature()`).
 * `aggregate` is the `aggregate_attestation` constructed in the previous section.
 
+## Phase 0 attestation subnet stability
+
+Because Phase 0 does not have shards and thus does not have Shard Committees, there is no stable backbone to the attestation subnets (`committee_index{subnet_id}_beacon_attestation`). To provide this stability, each validator must randomly select and remain subscribed to `RANDOM_SUBNETS_PER_VALIDATOR` attestation subnets. The lifetime of each random subscription must be _at least_ `EPOCHS_PER_RANDOM_SUBNET_SUBSCRIPTION`.
+
 ## How to avoid slashing
 
 "Slashing" is the burning of some amount of validator funds and immediate ejection from the active validator set. In Phase 0, there are two ways in which funds can be slashed: [proposer slashing](#proposer-slashing) and [attester slashing](#attester-slashing). Although being slashed has serious repercussions, it is simple enough to avoid being slashed all together by remaining _consistent_ with respect to the messages a validator has previously signed.