Add description to card metadata

Perplexity (PPL) is one of the most common metrics for evaluating language models.
It is defined as the exponentiated average negative log-likelihood of a sequence.

For more information, see https://huggingface.co/docs/transformers/perplexity

README.md

@@ -8,10 +8,18 @@ sdk_version: 3.0.2
 app_file: app.py
 pinned: false
 tags:
-- evaluate
-- metric
----
+  - evaluate
+  - metric
+description: >-
+  Perplexity (PPL) is one of the most common metrics for evaluating language
+  models.
+
+  It is defined as the exponentiated average negative log-likelihood of a
+  sequence.
 
+
+  For more information, see https://huggingface.co/docs/transformers/perplexity
+---
 # Metric Card for Perplexity
 
 ## Metric Description