Update governance, contributing, code of conduct docs (#3033)

* change from Inc. to LLC for Google

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

CODE-OF-CONDUCT.md

@@ -0,0 +1,3 @@
+## Community Code of Conduct
+
+gRPC follows the [CNCF Code of Conduct](https://github.com/cncf/foundation/blob/master/code-of-conduct.md).

CONTRIBUTING.md

@@ -1,6 +1,8 @@
 # How to contribute
 
-We definitely welcome your patches and contributions to gRPC!
+We definitely welcome your patches and contributions to gRPC! 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.
 
 If you are new to github, please start by reading [Pull Request howto](https://help.github.com/articles/about-pull-requests/)
 

GOVERNANCE.md

@@ -0,0 +1 @@
+This repository is governed by the gRPC organization's [governance rules](https://github.com/grpc/grpc-community/blob/master/governance.md).