Add GOVERNANCE.md to describe governance (#2830)

GOVERNANCE.md

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+# Governance
+
+M3 is owned by Uber, and is governed by its Technical Steering Committee (the
+"TSC").
+
+The current members of the TSC are:
+
+- @jimjag (Uber)
+- @martin-mao (Chronosphere)
+- @mway (Uber)
+- @prateek (Uber)
+- @robskillington (Chronosphere)
+
+While the TSC aims to operate as a consensus-based community, if any TSC
+decision requires a vote to move the proposal forward, decisions by vote require
+a majority vote of all TSC members.
+
+To reach the TSC directly, please email [email protected]. For administrative
+questions or issues, please contact [email protected]. To add the TSC to issues or
+pull requests, add the [TSC label](https://github.com/m3db/m3/labels/TSC).