Update readme to reflect new functionality

Author: alexhulbert

README.md

@@ -47,10 +47,37 @@ make build IMAGE=l2-builder
 # Build with development tools
 make build-dev IMAGE=flashbox-l1
 
+# Build dev image with serial console + password auth enabled
+make build-dev IMAGE=flashbox-l1 SERIAL_CONSOLE=true
+
 # View all available targets
 make help
 ```
 
+#### Dev image SSH access
+
+Dev images (`build-dev`) use SSH authorized keys for root login instead of a password. Before building, place your public key at:
+
+```
+mkosi.profiles/devtools/authorized_keys
+```
+
+This file is gitignored, so you can add a personal public key. For example:
+
+```bash
+cp ~/.ssh/id_ed25519.pub mkosi.profiles/devtools/authorized_keys
+```
+
+Once the image is running, SSH in as root:
+
+```bash
+ssh -p 2222 root@localhost
+```
+
+#### Serial console
+
+Pass `SERIAL_CONSOLE=true` to enable the serial console service and password authentication. This also sets a fixed root password (`dqSPjo4p`) for SSH login, so only use this for local development/debugging.
+
 ### Measuring TDX Boot Process
 
 **Export TDX measurements** for the built image: