feat(docs/op-stack): Add how to pause and unpause the bridge

docs/op-stack/src/.vuepress/config.js

@@ -186,6 +186,7 @@ module.exports = {
         children: [
           '/docs/security/faq.md',
           '/docs/security/policy.md',
+          '/docs/security/pause.md'
         ]
       },
     ],  // end of sidebar

docs/op-stack/src/docs/security/pause.md

+---
+title: Pause and Unpause the Bridge
+lang: en-US
+---
+
+
+## Why do it?
+
+The bridge is the way that assets move between blockchains.
+If there is a problem with the security of the blockchain, pausing the bridge is a way to mitigate the damage until the issue is resolved.
+
+## Who can do it?
+
+[`OptimismPortal`](https://github.com/ethereum-optimism/optimism/blob/develop/packages/contracts-bedrock/contracts/L1/OptimismPortal.sol) has an immutable `GUARDIAN`. 
+That address can call [`pause`](https://github.com/ethereum-optimism/optimism/blob/develop/packages/contracts-bedrock/contracts/L1/OptimismPortal.sol#L166-L170) and [`unpause`](https://github.com/ethereum-optimism/optimism/blob/develop/packages/contracts-bedrock/contracts/L1/OptimismPortal.sol#L175-L179).
+
+
+### Changing the guardian
+
+The guardian created by the setup script is the admin account.
+This is sufficient for testing, but for a production system you would want the guardian to be a multisig with trusted security council.
+
+The guardian is immutable, there is no way to change it while using the same contract.
+Luckily, it isn't supposed to be called directly, but through a proxy.
+[You can tell the proxy to go to a new contract](../build/tutorials/new-precomp.md).
+
+## Seeing it in action
+
+1. Set these environment variables
+
+   | Variable | Meaning
+   | - | - |
+   | `PRIV_KEY` | Private key for your ADMIN account
+   | `ADMIN_ADDR` | Address of the ADMIN account
+   | `PORTAL_ADDR` | Portal proxy address, get from `.../optimism/packages/contracts-bedrock/deployments/getting-started/OptimismPortalProxy.json`
+   | `GOERLI_RPC` | URL for an RPC to the L1 Goerli network 
+
+1.  For using Foundry, set `ETH_RPC_URL`.
+
+    ```sh
+    export ETH_RPC_URL=$GOERLI_RPC
+    ```
+
+1. Check the balance of the ADMIN account.
+   If it is too low you will not be able to submit transactions.
+
+   ```sh
+   cast balance $ADMIN_ADDR
+   ```
+
+1. Send a deposit to L2.
+
+   ```sh
+   cast send --private-key $PRIV_KEY --value 1gwei $PORTAL_ADDR
+   ```
+
+   Note the transaction hash.
+
+1. Pause the portal.
+
+   ```sh
+   cast send --private-key $PRIV_KEY $PORTAL_ADDR "pause()"
+   ```
+
+1. Send a deposit to L2.
+
+   ```sh
+   cast send --private-key $PRIV_KEY --value 1gwei $PORTAL_ADDR
+   ```
+
+   Note the transaction hash.
+
+1. Wait ten minutes and see which transaction(s) have been relayed using the [SDK](../build/sdk.md). 
+   Use [`getMessageStatus`](https://sdk.optimism.io/classes/crosschainmessenger#getMessageStatus) to get the information.
+
+
+
+1. Unpause the portal.
+
+   ```sh
+   cast send --private-key $PRIV_KEY $PORTAL_ADDR "pause()"
+   ```
\ No newline at end of file