Add maintainers list (#3992)

Author: khanhtc1202

MAINTAINERS.md

@@ -0,0 +1,12 @@
+# PipeCD maintainers
+
+| Name            | GitHub ID (not Twitter ID)                       | E-mail address for reporting security issues |
+|-----------------|--------------------------------------------------|----------------------------------------------|
+| Le Van Nghia    | [@nghialv](https://github.com/nghialv)           | (See GitHub profile)                         |
+| Tran Cong Khanh | [@khanhtc1202](https://github.com/khanhtc1202)   | (See GitHub profile)                         |
+| Kenta Nanao     | [@knanao](https://github.com/knanao)             | (See GitHub profile)                         |
+
+
+E-mail addresses are expected to be used for reporting security issues.
+Please use [GitHub Issues](https://github.com/pipe-cd/pipecd/issues) for reporting non-security issues,
+and [GitHub Discussions](https://github.com/pipe-cd/pipecd/discussions) or channel [#pipecd](https://cloud-native.slack.com/archives/C01B27F9T0X) on CNCF Slack for asking questions.