Update README.md

Added info and instructions on how to use the node.

README.md

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 # ComfyUI-RetroDiffusion-API-Node
-Retro Diffusion API node for ComfyUI
+
+A ComfyUI custom node for the [Retro Diffusion API](https://retrodiffusion.ai/)
+
+<img width="795" height="541" alt="comfyui_retrodiffusion_api_node showcase" src="https://github.com/user-attachments/assets/9f2c9634-74e6-4b61-a8f2-6e08132f2f4c" />
+
+## How to Install
+1. Go to [Retro Diffusion](https://retrodiffusion.ai/) and sign up for an account if you don't have one.
+2. Create an API key from the developer tools.
+3. In the root folder of your ComfyUI installation, create a `.env` file if it doesn't exist already.
+4. Add the following to your `.env`:
+```
+RD_API_KEY=your-key
+```
+5. Clone this repository into your `ComfyUI/custom_nodes` folder.
+6. Start ComfyUI.
+
+Now you can create a `Retro Diffusion API Node` in your workflows.
+
+## Features
+- Text-to-image and image-to-image
+- Outputs credit cost of the image and your remaining credits too
+- Supports `RD_Fast`, `RD_PLUS`, and `Animation` styles as of the latest commit
+
+More info on the Retro Diffusion API itself can be found here - https://github.com/Retro-Diffusion/api-examples
+
+## Notes
+- `RD_PLUS` and `Animation` styles are more expensive to generate than `RD_Fast`. 
+- Check the console if you get any errors, and feel free to create an issue.
+- There is an upload size limit for image-to-image, so keep that in mind (don't send large images as input).