Update SSH + Yubikey Usage

SSH-%2B-Yubikey-Usage.md

@@ -4,3 +4,15 @@ I use a Yubikey to store my ssh keys as a physical hardware key.
 
 1. Plug in the yubikey and make sure yubikey manager is installed (temporarily install it with `nix-shell -p yubikey-manager`)
 2. Run `ssh-keygen -K` to generate the `id_ed25519_sk_rk.pub` and `id_ed25519_sk_rk` files in the current directory.
+
+To add the key to the ssh-agent (ephemeral):
+```bash
+eval "$(ssh-agent -s)"
+ssh-add ~/.ssh/id_ed25519_sk_rk # Or whatever the path to the private key is
+```
+Or, to permanently add it to the agent, add the following to `~/.ssh/config`:
+```
+Host *
+  AddKeysToAgent yes
+  IdentityFile ~/.ssh/id_ed25519_sk_rk
+```