message: update README with Xeon Phi support note
diff: Processor Counter Monitor (PCM)
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  -Processor Counter Monitor (PCM) is an application programming interface (API) and a set of tools based on the API to monitor performance and energy metrics of Intel(r) Core(tm), Xeon(r) and Atom(tm) processors. PCM works on Linux, Windows, Mac OS X and FreeBSD operating systems.
  +Processor Counter Monitor (PCM) is an application programming interface (API) and a set of tools based on the API to monitor performance and energy metrics of Intel(r) Core(tm), Xeon(r), Atom(tm) and Xeon Phi(tm) processors. PCM works on Linux, Windows, Mac OS X and FreeBSD operating systems.
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