Create README.md (#6273)

I am adding a descriptive README.md file to my recently uploaded twitter classification model: shrugging-grace/tweetclassifier.

model_cards/shrugging-grace/tweetclassifier/README.md

+# shrugging-grace/tweetclassifier
+
+## Model description
+This model classifies tweets as either relating to the Covid-19 pandemic or not. 
+
+## Intended uses & limitations
+It is intended to be used on tweets commenting on UK politics, in particular those trending with the #PMQs hashtag, as this refers to weekly Prime Ministers' Questions.  
+
+#### How to use
+``LABEL_0`` means that the tweet relates to Covid-19
+
+``LABEL_1`` means that the tweet does not relate to Covid-19
+
+## Training data
+The model was trained on 1000 tweets (with the "#PMQs'), which were manually labeled by the author. The tweets were collected between May-July 2020. 
+
+### BibTeX entry and citation info
+
+This was based on a pretrained version of BERT. 
+
+@article{devlin2018bert,
+  title={Bert: Pre-training of deep bidirectional transformers for language understanding},
+  author={Devlin, Jacob and Chang, Ming-Wei and Lee, Kenton and Toutanova, Kristina},
+  journal={arXiv preprint arXiv:1810.04805},
+  year={2018}
+}