Add text-to-image task category

This PR updates the dataset card by adding `text-to-image` to the `task_categories` metadata. This addition accurately reflects that the dataset contains a large number of images generated by various text-to-image models (among others), making it highly relevant for research and benchmarking in the text-to-image domain.

README.md

@@ -1,13 +1,14 @@
 ---
 license: cc-by-nc-4.0
 task_categories:
-  - image-classification
+- image-classification
+- text-to-image
 library_name: datasets
 tags:
-  - ai-generated-content
-  - image-quality-assessment
-  - real-vs-fake
-  - multimodal
+- ai-generated-content
+- image-quality-assessment
+- real-vs-fake
+- multimodal
 ---
 
 # DANI: Discrepancy Assessing for Natural and AI Images