Add MAINTAINERS.md (#105)

Author: chalin

MAINTAINERS.md

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+# Active maintainers
+
+The active maintainers of this repository (in alphabetical order) are:
+
+- [bshaffer](https://github.com/bshaffer), Google LLC
+- [jamesward](https://github.com/jamesward), Google LLC
+- [lowasser](https://github.com/lowasser), Google LLC
+
+# Emeritus maintainers
+
+The emeritus maintainers of this repository (in alphabetical order) are:
+
+- None currently listed.
+
+# Notes
+
+- For governance guidelines, and how to become a maintainer, see
+  [grpc/grpc-community/governance.md][].
+- For general contribution guidelines, see
+  [grpc/grpc-community/CONTRIBUTING.md][].
+- If you were a maintainer and would like to add your name to the emeritus list,
+  submit a PR.
+
+[grpc/grpc-community/governance.md]: https://github.com/grpc/grpc-community/blob/master/governance.md
+[grpc/grpc-community/CONTRIBUTING.md]: https://github.com/grpc/grpc-community/blob/master/CONTRIBUTING.md