[example] add README to the rest-spark

dd-trace-examples/rest-spark/README.md

@@ -0,0 +1,62 @@
+## REST Spark
+
+This project provides a simple REST API using the [Spark web framework][1]. Even if the framework is not directly
+supported, manual instrumentation is used to trace one of the endpoints. A ``MongoClient`` is traced so that
+Mongo calls are part of the Spark trace.
+
+[1]: http://sparkjava.com/
+
+### Run the demo
+
+#### Prerequisites
+
+Be sure to build the project so that the latest version of ``dd-trace-java`` components are used. You can build
+all libraries and examples launching from the ``dd-trace-java`` root folder:
+```bash
+./gradlew clean shadowJar
+```
+
+Then you can prepare the distributable version of the ``rest-spark`` as follows:
+```bash
+cd dd-trace-examples/rest-spark
+./gradlew installDist
+```
+
+Then you can start all services via Docker:
+```bash
+DD_API_KEY=<your_datadog_api_key> docker-compose up -d
+```
+
+A valid ``DD_API_KEY`` is required to post collected traces to the Datadog backend.
+
+#### Run the application
+
+Launch the application using the run wrapper you've built during the ``installDist`` step:
+```bash
+JAVA_OPTS=-javaagent:../../dd-java-agent/build/libs/dd-java-agent-{version}.jar build/install/rest-spark/bin/rest-spark
+```
+
+``0.1.2-SNAPSHOT`` is an example of what ``{version}`` looks like.
+
+### Generate traces
+
+#### With your web browser
+
+Once the application runs. Go to the following url:
+
+* [http://localhost:4567/key/something][2]
+* [http://localhost:4567/key/something_else][3]
+
+Then get back to Datadog and wait a bit to see a trace coming.
+
+[2]: http://localhost:4567/key/something
+[3]: http://localhost:4567/key/something_else
+
+#### Auto-instrumentation with the `dd-trace-agent`
+
+The instrumentation is entirely done by the Java Agent which embed a set of rules that automatically recognizes &
+instruments:
+
+- The Mongo client
+
+The Java Agent embeds the [OpenTracing Java Agent](https://github.com/opentracing-contrib/java-agent).