Add readme

src/Extension/Propagator/XCloudTrace/README.md

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+[![Releases](https://img.shields.io/badge/releases-purple)](https://github.com/opentelemetry-php/extension-propagator-cloudtrace/releases)
+[![Source](https://img.shields.io/badge/source-extension--propagator--xcloudtrace-green)](https://github.com/open-telemetry/opentelemetry-php/tree/main/src/Extension/Propagator/XCloudTrace)
+[![Mirror](https://img.shields.io/badge/mirror-opentelemetry--php:extension--propagator--xcloudtrace-blue)](https://github.com/opentelemetry-php/extension-propagator-cloudtrace)
+[![Latest Version](http://poser.pugx.org/open-telemetry/extension-propagator-cloudtrace/v/unstable)](https://packagist.org/packages/open-telemetry/extension-propagator-cloudtrace/)
+[![Stable](http://poser.pugx.org/open-telemetry/extension-propagator-cloudtrace/v/stable)](https://packagist.org/packages/open-telemetry/extension-propagator-cloudtrace/)
+
+# OpenTelemetry Extension
+### XCloudTrace Propagator
+
+XCloudTrace is a propagator that supports the specification for the header "x-cloud-trace-context" used for trace context propagation across
+service boundaries. (https://cloud.google.com/trace/docs/setup#force-trace). OpenTelemetry PHP XCloudTrace Propagator Extension provides
+option to use it bi-directionally or one-way. One-way does not inject the header for downstream consumption, it only processes the incoming headers
+and returns the correct span context. It only attaches to existing X-Cloud-Trace-Context traces and does not create downstream ones.
+For one-way XCloudTrace:
+```text
+XCloudTracePropagator::getOneWayInstance()
+```
+
+For bi-directional XCloudTrace:
+```text
+XCloudTracePropagator::getInstance()
+```
+
+## Contributing
+
+This repository is a read-only git subtree split.
+To contribute, please see the main [OpenTelemetry PHP monorepo](https://github.com/open-telemetry/opentelemetry-php).