Add GOVERNANCE.md, CODE-OF-CONDUCT.md and CONTRIBUTING.md

Author: srini100

CODE-OF-CONDUCT.md

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+## Community Code of Conduct
+
+gRPC follows the [CNCF Code of Conduct](https://github.com/cncf/foundation/blob/master/code-of-conduct.md).

CONTRIBUTING.md

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+# How to contribute
+
+We definitely welcome patches and contributions to grpc.io! Please read the gRPC
+organization's [governance rules](https://github.com/grpc/grpc-community/blob/master/governance.md)
+and [contribution guidelines](https://github.com/grpc/grpc-community/blob/master/CONTRIBUTING.md) before proceeding.

GOVERNANCE.md

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+This repository is governed by the gRPC organization's [governance rules](https://github.com/grpc/grpc-community/blob/master/governance.md).