Documentation: add documentation on compiling for Android

Add documentation for cross-compiling on Android including:

() instructions on how to set the Android NDK environment
() how to cross-compile perf for Android
() how to install on an Android device/emulator, set the runtime
environment and run it

Signed-off-by: Irina Tirdea <irina.tirdea@intel.com>
Cc: David Ahern <dsahern@gmail.com>
Cc: Ingo Molnar <mingo@kernel.org>
Cc: Jiri Olsa <jolsa@redhat.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Paul Mackerras <paulus@samba.org>
Cc: Pekka Enberg <penberg@kernel.org>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Steven Rostedt <rostedt@goodmis.org>
Link: http://lkml.kernel.org/r/1349678613-7045-4-git-send-email-irina.tirdea@gmail.com


Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>