docs: add steps to revive expired client with governance proposal

docs/ibc/proposals.md

@@ -39,3 +39,48 @@ The substitute client is used as a "stand in" while the subject is on trial. It
 a substitute client *after* the subject has become frozen to avoid the substitute from also becoming frozen. 
 An active substitute client allows headers to be submitted during the voting period to prevent accidental expiry 
 once the proposal passes. 
+
+# How to revive an expired client with a governance proposal
+
+See also the relevant documentation: [ADR-026, IBC client recovery mechanisms](../architecture/adr-026-ibc-client-recovery-mechanisms.md)
+
+### Preconditions 
+- The chain is updated with ibc-go >= v1.1.0.
+- Recovery parameters are set to `true` for the Tendermint light client (this determines if a governance proposal can be used). If the recovery parameters are set to `false`, recovery will require modifying the migration code. In this case the easiest thing to do is probably just to mint back the tokens during migration in the next hub upgrade. 
+- The client identifier of an active client for the same connection.
+- The governance deposit.
+
+## Steps
+
+### Step 1
+
+Check if the client is attached to the expected `chain-id`. For example, for an expired Tendermint client on the Akash chain the client state looks like this on querying the client state:
+```
+{
+  client_id: 07-tendermint-146
+  client_state:
+  '@type': /ibc.lightclients.tendermint.v1.ClientState
+  allow_update_after_expiry: true
+  allow_update_after_misbehaviour: true
+  chain_id: akashnet-2
+}
+```
+The client is attached to the expected Akash `chain-id` and the recovery parameters (`allow_update_after_expiry` and `allow_update_after_misbehaviour`) are set to `true`.
+
+### Step 2
+
+If the chain has been updated to ibc-go >= v1.1.0, anyone can submit the governance proposal to recover the client by executing this via cli:
+
+```
+<binary> tx gov submit-proposal update-client <client_id> <active-counterparty-client in connection>
+```
+
+The `<active-counterparty-client in connection>` should be the client identifier of a currently active client which is relaying from the chain containing the expired client you are trying to revive. This means: whichever recommended/relayed channel is used right now could be reused to update the client.
+
+After this, it is just a question of who funds the governance deposit and if the chain in question votes yes.
+
+## Important considerations 
+
+Please note that v1.0.0 of ibc-go will not allow transactions to go to expired clients anymore, so please update to at least this version to prevent similar issues in the future.
+
+Please also note that if the client on the other end of the transaction is also expired, that client will also need to update. This process updates only one client.