Add readme for spark instrumentation (#14055)

Co-authored-by: otelbot <[email protected]>
Co-authored-by: Jay DeLuca <[email protected]>
Co-authored-by: Trask Stalnaker <[email protected]>

Author: 4 people

instrumentation/spark-2.3/README.md

@@ -0,0 +1,10 @@
+# Spark Instrumentation
+
+This instrumentation is for
+[Spark](https://github.com/perwendel/spark) -
+a tiny web framework for Java. It is also sometimes referred to as
+"Spark Java" to differentiate it from Apache Spark.
+
+This instrumentation is NOT for Apache Spark.
+
+This instrumentation does not emit telemetry on its own. Instead, it extracts the HTTP route and attaches it to SERVER spans and HTTP server metrics.