Updated README.md to fix pdebuild issue

Updated README.md to fix pdebuild issue

Fix issue #127.
This PR is to solve unexpected build issues when users try to run
`pdebuild` command to do Debian packaging of nnstreamer source.

**Changes proposed in this PR:**
1. Replaced **```** with **```bash**
2. Added how to create a base tarball with correct apt repository
3. Updated how to use pdebuild to avoid build errors
4. Used OTHERMIRROR instead of MIRRORSITE to handle lots of cases

**Self assessment:**
1. Local test with Ubuntu 16.04 (Xenial): Okay
2. GBS Build: Okay

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