Add trace generation papers from learning heuristics to README

May need a new section.

Papers are:
- Predicting Optimal Solution Cost with Bidirectional Stratified Sampling
- Learning Search-Space Specific Heuristics Using Neural Network (Issue #74)
- Learning Heuristic Functions in Classical Planning
- Neural Network Heuristics for Classical Planning: A Study of Hyperparameter Space
- Learning heuristic functions for large state spaces