Document L1_SERVERS configuration in README

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

README.md

@@ -20,6 +20,7 @@ export ARTDAG_DOMAIN=artdag.rose-ash.com
 export ARTDAG_USER=giles
 export ARTDAG_DATA=~/.artdag/l2
 export DATABASE_URL=postgresql://artdag:artdag@localhost:5432/artdag
+export L1_SERVERS=https://celery-artdag.rose-ash.com
 
 # Generate signing keys (required for federation)
 python setup_keys.py
@@ -96,6 +97,25 @@ Keys are stored in `$ARTDAG_DATA/keys/`:
 
 **Important**: Private keys are gitignored. Back them up securely. Losing them invalidates all your signatures.
 
+## L1 Renderers Configuration
+
+The `L1_SERVERS` environment variable defines which L1 rendering servers are available to users. Users can attach to these servers from the Renderers page to run effects and manage media.
+
+```bash
+# Single server (default)
+export L1_SERVERS=https://celery-artdag.rose-ash.com
+
+# Multiple servers (comma-separated)
+export L1_SERVERS=https://celery-artdag.rose-ash.com,https://renderer2.example.com,https://renderer3.example.com
+```
+
+When a user attaches to an L1 server:
+1. They're redirected to the L1's `/auth` endpoint with their auth token
+2. The L1 sets a local cookie, logging them in
+3. Their attachment is recorded in the `user_renderers` table
+
+Users can manage attachments at `/renderers`.
+
 ## Web UI
 
 The server provides a web interface:
@@ -106,6 +126,7 @@ The server provides a web interface:
 | `/assets` | Browse registered assets |
 | `/activities` | View published activities |
 | `/users` | List registered users |
+| `/renderers` | Manage L1 renderer connections |
 | `/asset/{name}` | Asset detail page |
 | `/login` | Login page |
 | `/register` | Registration page |