add docs

Author: mydea

docs/assets/node-sdk-trace-propagation.png

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+# How Trace Propagation Works in the JavaScript SDKs
+
+Trace propagation describes how and when traceId & spanId are set and send for various types of events.
+How this behaves varies a bit from Browser to Node SDKs.
+
+## Node SDKs (OpenTelemetry based)
+
+In the Node SDK and related OpenTelemetry-based SDKs, trace propagation works as follows:
+
+![node-sdk-trace-propagation-scenarios](./assets/node-sdk-trace-propagation.png)
+
+## Browser/Other SDKs
+
+TODO