perf script python: Add dict fields introduction to Documentation

Add a brief introduction about fields to perf-script-python.txt.

It should help python script developers in easily finding what fields
are supported.

Signed-off-by: Jin Yao <yao.jin@linux.intel.com>
Reviewed-by: Andi Kleen <ak@linux.intel.com>
Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Cc: Jin Yao <yao.jin@intel.com>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Kan Liang <kan.liang@linux.intel.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Link: http://lkml.kernel.org/r/1527843663-32288-4-git-send-email-yao.jin@linux.intel.com
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>

tools/perf/Documentation/perf-script-python.txt

@@ -610,6 +610,32 @@ Various utility functions for use with perf script:
   nsecs_str(nsecs) - returns printable string in the form secs.nsecs
   avg(total, n) - returns average given a sum and a total number of values
 
+SUPPORTED FIELDS
+----------------
+
+Currently supported fields:
+
+ev_name, comm, pid, tid, cpu, ip, time, period, phys_addr, addr,
+symbol, dso, time_enabled, time_running, values, callchain,
+brstack, brstacksym, datasrc, datasrc_decode, iregs, uregs,
+weight, transaction, raw_buf, attr.
+
+Some fields have sub items:
+
+brstack:
+    from, to, from_dsoname, to_dsoname, mispred,
+    predicted, in_tx, abort, cycles.
+
+brstacksym:
+    items: from, to, pred, in_tx, abort (converted string)
+
+For example,
+We can use this code to print brstack "from", "to", "cycles".
+
+if 'brstack' in dict:
+	for entry in dict['brstack']:
+		print "from %s, to %s, cycles %s" % (entry["from"], entry["to"], entry["cycles"])
+
 SEE ALSO
 --------
 linkperf:perf-script[1]