Added a link to Contribution guide in Readme (pytorch#40353)

Summary:
Added a link to `CONTRIBUTION.md` in `README.md` for easy reference.
Pull Request resolved: pytorch#40353

Differential Revision: D22167138

Pulled By: ezyang

fbshipit-source-id: fe7b7f190c8135fdd2e71696c1cf8d84bcd40fc6

README.md

@@ -353,6 +353,8 @@ We appreciate all contributions. If you are planning to contribute back bug-fixe
 If you plan to contribute new features, utility functions or extensions to the core, please first open an issue and discuss the feature with us.
 Sending a PR without discussion might end up resulting in a rejected PR, because we might be taking the core in a different direction than you might be aware of.
 
+To learn more about making a contribution to Pytorch, please see our [Contribution page](CONTRIBUTING.md).
+
 ## The Team
 
 PyTorch is a community driven project with several skillful engineers and researchers contributing to it.