Add model card with pipeline tag, library name and usage example

This PR adds a model card, linking the model to the paper page and Github repository. It also sets the relevant `pipeline_tag` and `library_name`.
I've added a brief description of the model and an example of how to use it.

README.md

@@ -1,3 +1,20 @@
----
-license: mit
----
+---
+license: mit
+pipeline_tag: image-to-video
+library_name: diffusers
+---
+
+This model performs image-to-video generation based on the paper [FlexWorld: Progressively Expanding 3D Scenes for Flexible-View Synthesis](https://arxiv.org/abs/2503.13265).
+
+Project page: https://ml-gsai.github.io/FlexWorld
+
+Code: https://github.com/ml-gsai/FlexWorld
+
+## Usage Example
+
+A basic example of generating a static scene video given an image and a camera trajectory:
+
+```bash
+# You can utilize our CamPlanner class to freely construct the desired trajectory at line 13 in the `video_generate.py` file.
+python video_generate.py --input_image_path ./assets/room.png --output_dir ./results-single-traj
+```