Add README

Author: tylerbenson

dd-java-agent/agent-jmxfetch/src/main/resources/datadog/trace/agent/jmxfetch/README

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+# Metric Configs
+
+These files are copied directly from [integrations-core](https://github.com/DataDog/integrations-core/search?q=jmx_metrics&unscoped_q=jmx_metrics)
+where the only modification is updating the file name.
+
+These will be bundled in `dd-java-agent.jar`. Due to limitations in Java, it is non-trivial
+to get all these files from within the jar without knowing their names.
+Consequently, we list out each integration in `metricconfigs.txt` so the agent can reference them.