modifications to README

README.md

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 # grmpy
 
-This repository will serve in our efforts to restart the **grmpy** package
+``grmpy``  is an open-source Python package for the simulation and estimation of generalized Roy Model. Its main purpose is to serve as a teaching tool to promote the conceptual framework provided by the generalized Roy model to illustrate a variety of issues in the econometrics of policy evaluation. The main references are:
+
+*  Heckman, J. J., and Vytlacil, E. J. (2007a). [Econometric evaluation of social programs, Part I: Causal effects, structural models and econometric policy evaluation.](http://ac.els-cdn.com/S1573441207060709/1-s2.0-S1573441207060709-main.pdf?_tid=b933f5c8-6bbe-11e7-8ae8-00000aacb35d&acdnat=1500385435_c69182d36b79b66bbce5f5a7c593617c) In Heckman, J. J., and Leamer, E. E., editors, *Handbook of Econometrics*, volume 6B, pages 4779–4874. Elsevier
+Science, Amsterdam, Netherlands.
+
+* Heckman, J. J., and Vytlacil, E. J. (2007b). [Econometric evaluation of social programs, Part II: Using the marginal treatment effect to organize alternative economic estimators to evaluate social programs and to forecast their effects in new environments.](http://ac.els-cdn.com/S1573441207060710/1-s2.0-S1573441207060710-main.pdf?_tid=5ccb4ace-6bbf-11e7-807b-00000aab0f26&acdnat=1500385710_c3706f18138fabe356b0f3ebddd75670) In Heckman, J. J. and Leamer, E. E., editors, *Handbook of Econometrics*, volume 6B, pages 4875–5144. Elsevier Science, Amsterdam, Netherlands.
+
+Please visit our [online documentation](http://grmpy.readthedocs.io/) for details.
 
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