[Documentation] Added development guide of nnstreamer

[Documentation] Added development guide of nnstreamer

This PR is to append some mark-down documents to develop
nnstreamer source code consistently.

Original documents is mirrored from TAOS-CI repository.
Thane, modified the document for nnstreamer project.

**Changes proposed in this PR:**

* Version 3:
1. Removed Python rule
2. Updated how to run gst-indent before submitting PR

* Version 2:
1. Replaced a manual indent command with ./common/gst-indent script

* Version 1:
1. Added coding convention document
2. Added how to contribute code
3. Added generation script to create TOC automatically
4. Added how to archive large data for evaluation
   (In this case, another repository has to be created for only large data.)

**Self assessment:**
1. gbs build: okay
2. markd-down generation: okay

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