[model_cards] Add card by @lvwerra

(the current way to submit a model card to have it displayed on the website is to open a PR on the `transformers` repo itself)

Thanks for sharing!

model_cards/lvwerra/gpt2-medium-taboo/README.md

+# GPT-2 (medium) Taboo
+
+## What is it?
+A fine-tuned GPT-2 version for Taboo cards generation.
+
+## Training setting
+
+The model was trained on ~900 Taboo cards in the following format for 100 epochs:
+```
+Describe the word Glitch without using the words Problem, Unexpected, Technology, Minor, Outage.
+````
+