Added example commands to `nettacker.py` to address #740.

[x] I have followed the Contributor Guidelines.
[x] The code has been thoroughly tested in my local development environment with flake8 and pylint.
[x] The code is Python 3 compatible.
[x] The code follows the PEP8 styling guidelines with 4 spaces indentation.
[x] This Pull Request relates to only one issue or only one feature
[x] I have referenced the corresponding issue number in my commit message
[x] I have added the relevant documentation.
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OWASP / Nettacker