Add VERSIONING doc (#2490)

VERSIONING.md

@@ -0,0 +1,33 @@
+# OpenTelemetry Java Instrumentation Versioning
+
+## Compatibility requirements
+
+Artifacts in this repository follow the same compatibility requirements described in
+https://github.com/open-telemetry/opentelemetry-java/blob/main/VERSIONING.md#compatibility-requirements
+.
+
+EXCEPT for the following incompatible changes which are allowed in stable artifacts in this
+repository:
+
+* Changes to the telemetry produced by instrumentation
+  (there will be some guarantees about telemetry stability in the future, see discussions
+  in https://github.com/open-telemetry/opentelemetry-specification/issues/1301)
+* Changes to configuration properties that contain the word `experimental`
+* Changes to configuration properties under the namespace `otel.javaagent.testing`
+
+This means that:
+
+* Changes to configuration properties (other than those that contain the word `experimental`
+  or are under the namespace `otel.javaagent.testing`) will be considered breaking changes
+  (unless they only affect telemetry produced by instrumentation)
+
+## Stable vs alpha
+
+See https://github.com/open-telemetry/opentelemetry-java/blob/main/VERSIONING.md#stable-vs-alpha
+
+IN PARTICULAR:
+
+Not all of our artifacts are published as stable artifacts - any non-stable artifact has the suffix
+`-alpha` on its version. NONE of the guarantees described above apply to alpha artifacts. They may
+require code or environment changes on every release and are not meant for consumption for users
+where versioning stability is important.