Migrate model card from transformers-repo

Read announcement at https://discuss.huggingface.co/t/announcement-all-model-cards-will-be-migrated-to-hf-co-model-repos/2755
Original file history: https://github.com/huggingface/transformers/commits/master/model_cards/twmkn9/distilbert-base-uncased-squad2/README.md

README.md

@@ -0,0 +1,45 @@
+This model is [Distilbert base uncased](https://huggingface.co/distilbert-base-uncased) trained on SQuAD v2 as:
+
+```
+export SQUAD_DIR=../../squad2
+python3 run_squad.py 
+    --model_type distilbert 
+    --model_name_or_path distilbert-base-uncased
+    --do_train 
+    --do_eval 
+    --overwrite_cache 
+    --do_lower_case 
+    --version_2_with_negative 
+    --save_steps 100000 
+    --train_file $SQUAD_DIR/train-v2.0.json 
+    --predict_file $SQUAD_DIR/dev-v2.0.json 
+    --per_gpu_train_batch_size 8 
+    --num_train_epochs 3 
+    --learning_rate 3e-5 
+    --max_seq_length 384 
+    --doc_stride 128 
+    --output_dir ./tmp/distilbert_fine_tuned/
+```
+
+Performance on a dev subset is close to the original paper:
+
+```
+Results: 
+{
+    'exact': 64.88976637051661, 
+    'f1': 68.1776176526635, 
+    'total': 6078, 
+    'HasAns_exact': 69.7594501718213, 
+    'HasAns_f1': 76.62665295288285, 
+    'HasAns_total': 2910, 
+    'NoAns_exact': 60.416666666666664, 
+    'NoAns_f1': 60.416666666666664, 
+    'NoAns_total': 3168, 
+    'best_exact': 64.88976637051661, 
+    'best_exact_thresh': 0.0, 
+    'best_f1': 68.17761765266337, 
+    'best_f1_thresh': 0.0
+}
+```
+
+We are hopeful this might save you time, energy, and compute. Cheers!