Tools/gst-instruments: Added how to profile a bottleneck among elements

Tools/gst-instruments: Added how to profile a bottleneck among elements

Fixed issue #132 (Epic: Performance- Profiling)

This commit is to append how to use a gst-instruments tool in order to
do a profiling for performance optimization of NNStreamer.
This guide is experimented on Ubuntu 16.04 x86_64 distribution.

**Changes proposed in this PR:**
1. Added gst-top to display a performance report in real time such as top, perf-top
2. Added gst-report-1.0 to generate the trace file the below two goals as following:
   a. It display CPU usage, time usage, and execution time amon the elements.
   b. It generate a performance graph from a GStreamer pipeline.
3. Added screenshot of gst-top, gst-report, and generating pipeline graph

Signed-off-by: Geunsik Lim <geunsik.lim@samsung.com>