adding custom genesis guidance

Author: sbvegan

pages/notices/upgrade-15.mdx

@@ -121,6 +121,10 @@ These following steps are necessary for every node operator:
     The override flag for op-node was missed and will be added in the mainnet release. If you utilize the override flags, please just set the activation time in the rollup configuration file. 
   </Callout>
 
+  <Callout type="warning">
+    If you use custom genesis chain configuration, you need to set the `pragueTime` to the same value as the `isthmusTime`. It is not automatically set, this automatically happens for chains using the network flags and also when using the override flags.
+  </Callout>
+
   *   **Option 1:** Set the activation time in the `rollup.json` for `op-node`. You will still need to set the `override.isthmus` flag in `op-geth` if you use this option. Please note that the chain configuration file is subject to a stricter format and needs to contain the `chain_op_config` outlined in the `op-node/v1.11.0` [release notes](https://github.com/ethereum-optimism/optimism/releases/tag/op-node%2Fv1.11.0).
   *   **Option 2:** Set the activation time via overrides (CLI) in both `op-node` and `op-geth`. These will need to be set on `op-node` and `op-geth` for the sequencer and all other nodes.