Add GPL License; Update README and CITATION

Author: cr-xu

CITATION.cff

@@ -19,7 +19,6 @@ authors:
     orcid: "https://orcid.org/0000-0002-7498-7640"
 
 title: "Tutorial at the RL4AA'24 Workshop"
-version: 1.0.0
 date-released: 2024-03-27
 url: "https://github.com/RL4AA/rl4aa24-tutorial"
 doi: 10.5281/zenodo.10886639