Add docs for perf data collection

docs/performance_data.md

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+Performance Data
+================
+
+AWX has the ability to collect performance data on job runs.
+
+The following data is collected periodically (with a default interval of every 0.25 seconds):
+* CPU usage
+* Memory usage
+* PID count
+
+The data is stored under `/var/log/tower/playbook_profiling`. A new folder is created for each job run. The folder's name is set to the job's ID.
+
+Performance data collection is not enabled by default. To enable performance data collection on all jobs, set AWX_RESOURCE_PROFILING_ENABLED to true.
+
+The frequency with which data is collected can be set using:
+* AWX_RESOURCE_PROFILING_CPU_POLL_INTERVAL
+* AWX_RESOURCE_PROFILING_MEMORY_POLL_INTERVAL
+* AWX_RESOURCE_PROFILING_PID_POLL_INTERVAL