Add COMPATIBILITY.md to describe version compatibility (#2829)

COMPATIBILITY.md

@@ -0,0 +1,25 @@
+# Compatibility
+
+This document describes versioning compatibility guarantees. Any compatibility
+outlined here will be maintained for at least the current major version.
+Additional guarantees may be added within the same major version, but existing
+guarantees may only be modified or removed as part of a major version change.
+
+If you discover a breaking change, please [open an issue][issue].
+
+[issue]: https://github.com/m3db/m3/issues/new
+
+## v1.0
+
+As of version 1.0, m3db/m3 and its components are guaranteed to have **binary**,
+**configuration**, **wire**, and **transport API** compatibility:
+
+| Compatibility | Scope | Guarantee |
+| :------------ | :---- | :-------- |
+| binary | All released executable components. | All executable components will maintain their current functionality. Components may have *additional* functionality added, but no existing functionality will be changed or removed. |
+| configuration | All configuration serialization for binary components. | All configuration will maintain serialization compatibility. New configuration properties may be added, but existing properties will not be renamed, removed, re-typed, or changed semantically or behaviorally. |
+| wire | All on-the-wire encoding for data to/from binary components. | All wire encoding (e.g. Protobuf, Thrift) will maintain wire compatibility. Any changes to IDLs or other encodings will be backwards-compatible. |
+| transport API | All exposed transport APIs for communication to/from binary components. | All transport APIs (e.g. m3msg, Thrift, HTTP+JSON) will maintain their current surface area, definitions, semantics, and behavior. Any changes to APIs will be additive or done semantically and with wire compatibility. |
+
+This version does **not** guarantee library-level compatibility. Users are not
+encouraged to integrate libraries from m3db/m3 directly: use at your own risk.