Add description to card metadata

IoU is the area of overlap between the predicted segmentation and the ground truth divided by the area of union
between the predicted segmentation and the ground truth. For binary (two classes) or multi-class segmentation,
the mean IoU of the image is calculated by taking the IoU of each class and averaging them.

README.md

@@ -1,6 +1,6 @@
 ---
 title: Mean IoU
-emoji: 🤗 
+emoji: 🤗
 colorFrom: blue
 colorTo: red
 sdk: gradio
@@ -8,10 +8,18 @@ sdk_version: 3.0.2
 app_file: app.py
 pinned: false
 tags:
-- evaluate
-- metric
----
+  - evaluate
+  - metric
+description: >-
+  IoU is the area of overlap between the predicted segmentation and the ground
+  truth divided by the area of union
+
+  between the predicted segmentation and the ground truth. For binary (two
+  classes) or multi-class segmentation,
 
+  the mean IoU of the image is calculated by taking the IoU of each class and
+  averaging them.
+---
 # Metric Card for Mean IoU