Add documentation EUD model training.

docs/training.md

@@ -43,7 +43,27 @@ Examples (for clarity without training/validation data paths):
     ```bash
     combo --mode train --targets head,deprel --features token,char,upostag
     ```
-  
+
+## Enhanced UD
+
+Training a model with Enhanced UD prediction **requires** data pre-processing.
+
+```bash
+combo --mode train \
+      --training_data_path your_preprocessed_training_path \
+      --validation_data_path your_preprocessed_validation_path \
+      --targets feats,upostag,xpostag,head,deprel,lemma,deps \
+      --config_path config.graph.template.jsonnet
+```
+### Data pre-processing
+Download data from [IWPT20 Shared Task](https://universaldependencies.org/iwpt20/data.html).
+It contains `enhanced_collapse_empty_nodes.pl` script which is required as pre-processing step.
+Apply this script to training and validation data.
+
+```bash
+perl enhanced_collapse_empty_nodes.pl training.conllu > training.fixed.conllu
+``` 
+
 ## Configuration
 
 ### Advanced