perf symbols: Add description of JIT interface

Add a description of the JIT interface in the perf symbol resolution
code. I reverse engineered the format from the source.
Signed-off-by: Andi Kleen <ak@linux.intel.com>
Acked-by: Pekka Enberg <penberg@cs.helsinki.fi>
Cc: Pekka Enberg <penberg@cs.helsinki.fi>
Link: http://lkml.kernel.org/r/1344526260-18721-1-git-send-email-andi@firstfloor.orgSigned-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>

tools/perf/Documentation/jit-interface.txt

+perf supports a simple JIT interface to resolve symbols for dynamic code generated
+by a JIT.
+
+The JIT has to write a /tmp/perf-%d.map  (%d = pid of process) file
+
+This is a text file.
+
+Each line has the following format, fields separated with spaces:
+
+START SIZE symbolname
+
+START and SIZE are hex numbers without 0x.
+symbolname is the rest of the line, so it could contain special characters.
+
+The ownership of the file has to match the process.