Add DD_DURABLE_CROSS_INVOCATION_TRACING_ENABLED explanation in README.md

Author: joeyzhao2018

README.md

@@ -30,6 +30,7 @@ Besides the environment variables supported by dd-trace-py, the datadog-lambda-p
 | DD_CAPTURE_LAMBDA_PAYLOAD | [Captures incoming and outgoing AWS Lambda payloads][1] in the Datadog APM spans for Lambda invocations. | `false` |
 | DD_CAPTURE_LAMBDA_PAYLOAD_MAX_DEPTH | Determines the level of detail captured from AWS Lambda payloads, which are then assigned as tags for the `aws.lambda` span. It specifies the nesting depth of the JSON payload structure to process. Once the specified maximum depth is reached, the tag's value is set to the stringified value of any nested elements beyond this level.  <br> For example, given the input payload: <pre>{<br>  "lv1" : {<br>    "lv2": {<br>      "lv3": "val"<br>    }<br>  }<br>}</pre> If the depth is set to `2`, the resulting tag's key is set to `function.request.lv1.lv2` and the value is `{\"lv3\": \"val\"}`. <br> If the depth is set to `0`, the resulting tag's key is set to `function.request` and value is `{\"lv1\":{\"lv2\":{\"lv3\": \"val\"}}}` | `10` |
 | DD_EXCEPTION_REPLAY_ENABLED | When set to `true`, the Lambda will run with Error Tracking Exception Replay enabled, capturing local variables. | `false` |
+| DD_DURABLE_CROSS_INVOCATION_TRACING_ENABLED | For AWS Durable functions, the tracer creates extra checkpoints named `_datadog_{N}` to propagate trace context across function invocations, keeping spans from multiple invocations in one intact trace for each durable execution. | `true` |
 
 
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