Add MAINTAINERS.md file (#477)

* Add MAINTAINERS.md file

Author: jtattermusch

MAINTAINERS.md

@@ -0,0 +1,17 @@
+This page lists all active maintainers of this repository. If you were a
+maintainer and would like to add your name to the Emeritus list, please send us a
+PR.
+
+See [GOVERNANCE.md](https://github.com/grpc/grpc-community/blob/master/governance.md)
+for governance guidelines and how to become a maintainer.
+See [CONTRIBUTING.md](https://github.com/grpc/grpc-community/blob/master/CONTRIBUTING.md)
+for general contribution guidelines.
+
+## Maintainers (in alphabetical order)
+- [apolcyn](https://github.com/apolcyn), Google LLC
+- [davidfowl](https://github.com/davidfowl), Microsoft Corporation
+- [JamesNK](https://github.com/JamesNK), Microsoft Corporation
+- [jtattermusch](https://github.com/jtattermusch), Google LLC
+- [JunTaoLuo](https://github.com/JunTaoLuo), Microsoft Corporation
+
+## Emeritus Maintainers (in alphabetical order)