perf tools: Add documentation for perf.data on disk format

Add some documentation for the on disk format of perf.data. This is not
documenting the actual perf events -- which are documented in
perf_event.h -- but just the additional headers that perf record adds
around them when writing the data to disk.
Signed-off-by: Andi Kleen <ak@linux.intel.com>
Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: Jiri Olsa <jolsa@kernel.org>
Link: http://lkml.kernel.org/r/1466800885-12974-1-git-send-email-andi@firstfloor.orgSigned-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>