Add ComfyUI setup notes for the Create feature

comfyui/README.md

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+# ComfyUI workflow for the "Create" feature
+
+DrawIt's **Create** feature sends a fixed coloring-book prompt to a self-hosted [ComfyUI](https://github.com/comfyanonymous/ComfyUI)
+server and turns the result into a lesson. Nothing leaves your machine.
+
+## Setup
+
+1. Run ComfyUI (default: `http://127.0.0.1:8188`).
+2. Set `COMFYUI_URL` in `.env.local` (blank = feature disabled).
+3. Provide a workflow exported in **API format** (ComfyUI → Settings → enable dev mode →
+   "Save (API format)") and point `COMFYUI_WORKFLOW` at it.
+
+`coloring-book.workflow_api.json` here is the **stock SD1.5 text-to-image workflow** as a starting
+point. DrawIt injects its prompt into the positive-prompt node and a random seed into the sampler node:
+
+- `COMFYUI_PROMPT_NODE` (default `6`) — the positive `CLIPTextEncode` node.
+- `COMFYUI_SEED_NODE` (default `3`) — the `KSampler` node.
+
+## Tips for clean line art (matters — we vectorize it with potrace)
+
+The cleaner and bolder the outlines, the better the trace lessons look. Recommended:
+
+- Use a **line-art / coloring-book LoRA or model** if you have one, and bump `steps`.
+- Keep the negative prompt rejecting color/shading/background clutter (already set).
+- The image is auto-thresholded to pure black/white and fitted to the lesson canvas, so aim for
+  **bold, fully-closed outlines on a white background** — thin or broken lines vectorize poorly.
+
+Edit `ckpt_name` to a checkpoint you actually have installed.