added contributing page

CONTRIBUTING.MD

+Pytorch-widedeep is being developed and used by many active community members. Your help is very valuable to make it better for everyone.
+
+- **[TBA]** Check for the [Roadmap](https://github.com/jrzaurin/pytorch-widedeep/projects/1) or [Open an issue](https://github.com/microsoft/jrzaurin/pytorch-widedeep/issues) to report problems or recommend new features and submit a draft pull requests, which will be changed to pull request after intial review
+- Contribute to the [tests](https://github.com/jrzaurin/pytorch-widedeep/tree/master/tests) to make it more reliable.
+- Contribute to the [documentation](https://github.com/jrzaurin/pytorch-widedeep/tree/master/docs) to make it clearer for everyone.
+- Contribute to the [examples](https://github.com/jrzaurin/pytorch-widedeep/tree/master/examples) to share your experience with other users.
+- Join the dicussion on [slack](https://join.slack.com/t/pytorch-widedeep/shared_invite/zt-soss7stf-iXpVuLeKZz8lGTnxxtHtTw)
\ No newline at end of file

README.md

@@ -24,8 +24,6 @@ using wide and deep models.
 
 **Experiments and comparisson with `LightGBM`**: [TabularDL vs LightGBM](https://github.com/jrzaurin/tabulardl-benchmark)
 
-**Slack**: if you want to contribute or just want to chat with us, join [slack](https://join.slack.com/t/pytorch-widedeep/shared_invite/zt-soss7stf-iXpVuLeKZz8lGTnxxtHtTw)
-
 The content of this document is organized as follows:
 
 1. [introduction](#introduction)
@@ -307,6 +305,10 @@ of the package and its functionalities.
 pytest tests
 ```
 
+### How to Contribute
+
+Check [CONTRIBUTING](https://github.com/jrzaurin/pytorch-widedeep/CONTRIBUTING.MD) page.
+
 ### Acknowledgments
 
 This library takes from a series of other libraries, so I think it is just