Tools: Add how to use GST_DEBUG to get debug messages

Tools: Add how to use GST_DEBUG to get debug messages

Fixed issue #132 (Epic: Performance - Debugging)

This commit is to append how to use environment variable GST_DEBUG to
get more debug messages for debugging of NNStreamer.

**Changes proposed in this PR:**
* Version 2:
1. Case study: Tracing Gstreamer plugins with GST_DEBUG

* Version 1:
1. Added how to use GST_DEBUG
2. Added execution screenshot of xdot
3. Updated the existing statement for readability
4. Updated GST_DEBUG_DUMP_DOT_DIR usage

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