added README.md and moved otlp_aws_span_exporter into its own folder

Author: liustve

aws-opentelemetry-distro/src/amazon/opentelemetry/distro/aws-span-exporter/README.md

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+# ADOT X-Ray OTLP Trace Export Configuration
+
+This guide explains how to automatically configure ADOT environment variables for exporting traces to [AWS X-Ray OTLP endpoint](https://docs.aws.amazon.com/xray/latest/devguide/xray-opentelemetry.html)
+
+## Prerequisites
+
+1. Transaction Search must be enabled in order to send spans to the Xray OTLP endpoint. See [this doc](https://docs.aws.amazon.com/AmazonCloudWatch/latest/monitoring/CloudWatch-Transaction-Search-getting-started.html) on how to enable Transaction Search.
+
+2. Ensure the AWS role in your application environment has [AWSXRayWriteOnlyAccess](https://docs.aws.amazon.com/aws-managed-policy/latest/reference/AWSXrayWriteOnlyAccess.html) managed policy attached.
+
+## Environment Variables
+
+Set the following environment variables to **PROPERLY** configure trace export with SigV4 authentication:
+
+```shell
+# Set X-Ray endpoint (replace AWS_REGION with your region)
+OTEL_EXPORTER_OTLP_TRACES_ENDPOINT=https://xray.<AWS_REGION>.amazonaws.com/v1/traces
+
+# Configure OTLP export protocol
+OTEL_EXPORTER_OTLP_PROTOCOL=http/protobuf
+
+OTEL_TRACES_EXPORTER=otlp
+OTEL_METRICS_EXPORTER=none
+OTEL_LOGS_EXPORTER=none
+OTEL_RESOURCE_ATTRIBUTES="service.name=<YOUR_SERVICE_NAME>"
+
+# Disable application signals (they will be generated in CloudWatch backend)
+OTEL_AWS_APPLICATION_SIGNALS_ENABLED=false