fix(docs): instructions for sortition keygen

docs/run-node/6-flare-systems-provider.mdx

@@ -150,7 +150,13 @@ Accounts for Fast Updates submission need to be generated and funded with gas fe
 These can be any accounts **not used** for the five accounts in [register addresses](#register-addresses).
 You are recommended to use **three** separate accounts to avoid nonce conflicts.
 
-Register the generated sortition key via the `EntityManager` contract by calling [`registerPublicKey`](/network/fsp/solidity-reference/IEntityManager#registerpublickey) with your identity address.
+When registering the sortition key using the explorer, you will need to manually create a signature:
+
+```bash
+docker run --rm ghcr.io/flare-foundation/fast-updates/go-client:latest keygen --key <sortitionKeyPrivateKey> --address <identityAddress>
+```
+
+Register the generated sortition key and signature via the `EntityManager` contract by calling [`registerPublicKey`](/network/fsp/solidity-reference/IEntityManager#registerpublickey) with your identity address, use the signature generated by the previous command as the input to `_verificationData`.
 
 <details>
 <summary>Automated registration on testnets.</summary>