docs: nixops-krops migration guide

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+# Tutorial: Moving from a NixOps to a Krops deployment
+
+1. Add a new ssh key to your nix-bitcoin node
+
+   Krops doesn't automatically generate ssh keys like NixOps, instead you add your own.
+
+   If you don't have a ssh key yet
+
+   ```
+   ssh-keygen -t ed25519 -f ~/.ssh/bitcoin-node
+   ```
+
+   Edit `configuration.nix`
+
+   ```
+   users.users.root = {
+     openssh.authorizedKeys.keys = [
+       "<contents of ~/.ssh/bitcoin-node.pub or existing .pub key file>"
+     ];
+   };
+   ```
+
+   Deploy new key
+
+   ```
+   nixops deploy -d bitcoin-node
+   ```
+
+2. Update your nix-bitcoin, depending on your setup either with `fetch-release` or `git`. Make sure you are at least on `v0.0.41`.
+
+3. Pull the latest nix-bitcoin source
+
+    ```
+    cd ~/nix-bitcoin
+    git pull
+    ```
+
+4. Copy new and updated files into your deployment folder
+
+    ```
+    cd <deployment directory, for example `~/nix-bitcoin-node`>
+    cp -r ~/nix-bitcoin/examples/{krops,shell.nix} .
+    ```
+
+5. Edit your ssh config
+
+    ```
+    nano ~/.ssh/config
+    ```
+
+    and add the node with an entry similar to the following (make sure to fix `Hostname` and `IdentityFile`):
+
+    ```
+    Host bitcoin-node
+        # FIXME
+        Hostname NODE_IP_ADDRESS_OR_HOST_NAME_HERE
+        User root
+        PubkeyAuthentication yes
+        # FIXME
+        IdentityFile <ssh key from step 1 or path to existing key>
+        AddKeysToAgent yes
+    ```
+
+6. Make sure you are in the deployment directory and edit `krops/deploy.nix`
+
+    ```
+    nano krops/deploy.nix
+    ```
+
+    Locate the `FIXME` and set the target to the name of the ssh config entry created earlier, i.e. `bitcoin-node`.
+
+    Note that any file imported by your `configuration.nix` must be copied to the target machine by krops.
+    For example, if there is an import of `networking.nix` you must add it to `extraSources` in `krops/deploy.nix` like this:
+    ```
+    extraSources = {
+        "hardware-configuration.nix".file = toString ../hardware-configuration.nix;
+        "networking.nix".file = toString ../networking.nix;
+    };
+    ```
+
+7. If `lnd` or `joinmarket` is enabled on your node, run the commmand
+   ```
+   nix-shell --run 'nix-instantiate --eval -E "
+     (import <nixpkgs/nixos> {
+       configuration = { lib, ... }: {
+         imports = [ ./configuration.nix ];
+         nix-bitcoin.configVersion = lib.mkDefault \"0.0.31\";
+         nix-bitcoin.secretsSetupMethod = lib.mkForce \"manual\";
+       };
+     }).vm.outPath
+   "'
+   ```
+   and follow the migration instructions from the error message.
+
+8. Optional: Disallow substitutes
+
+    You may have been building nix-bitcoin "without substitutes" to avoid pulling in binaries from the Nix cache. If you want to continue doing so, you have to add the following line to the `configuration.nix`:
+    ```
+    nix.extraOptions = "substitute = false";
+    ```
+
+    If the build process fails for some reason when deploying with `krops-deploy` (see later step), it may be difficult to find the cause due to the missing output.
+    In that case, it is possible to SSH into the target machine and run
+    ```
+    nixos-rebuild -I /var/src switch
+    ```
+
+9. Deploy with krops
+
+    ```
+    nix-shell --run krops-deploy
+    ```
+    Remove the old secrets directory. For krops deployments, secrets are always
+    located at `/var/src/secrets`.
+    ```
+    ssh bitcoin-node 'rm -rf /secrets'
+    ```
+
+9. You can now access `bitcoin-node` via ssh
+
+    ```
+    ssh operator@bitcoin-node
+    ```
+
+10. You can remove the remaining traces of nixops as follows:
+    ```
+    nix-shell
+    nix run -f '<nix-bitcoin>' nixops19_09 -c nixops delete -d bitcoin-node --force
+    git rm -r nixops
+    ```