Java Agent Migration - add steps for shim and classpath (#39)

* add step to migration to add shim and update classpath

* md formatting

Author: brianashby

migration.md

@@ -46,6 +46,14 @@ distribution of Splunk Distribution of OpenTelemetry Java Instrumentation:
    setting.
 4. In your application startup script, replace `-javaagent:./signalfx-tracing.jar`
    with `-javaagent:/path/to/splunk-otel-javaagent-all.jar`.
+5. If you manually instrumented any code with an OpenTracing tracer, expose
+   the OpenTelemetry tracer as an implementation of an OpenTracing tracer with
+   the OpenTracing Shim. For more information, see
+   [OpenTelemetry - OpenTracing Shim](https://github.com/open-telemetry/opentelemetry-java/tree/master/opentracing-shim).
+   If you use another API for manual instrumentation, such as for the `@Trace`
+   annotation in the SignalFx Java Agent, ensure it's in your application's
+   `classpath` as well. For an example of what this looks like, see this
+   [SignalFx Java Agent example application](https://github.com/signalfx/tracing-examples/blob/master/signalfx-tracing/signalfx-java-tracing/okhttp-and-jedis/src/main/java/com/signalfx/tracing/examples/javaagent/App.java).
 
 ## Changes in functionality