Enhance the requirements setup and add a basic contributing guide

[greyli]

This PR continues #153, as I addressed in that PR:

By the way, I think these requirements should only list dev dependencies, if you agree, I will update them to remove the repeat dependencies that are already defined in the setup.py. Since we normally use the following commands to start to develop the project:

pip install -e .
pip install -r requirements/dev.txt

So this PR implemented that proposal:

• Remove the dependencies that already defined in setup.py install_requires
• Split requirements into dev/example/test
• Update the GitHub workflows

This PR also adds a simple contributing section to the README, while a complete contributing guide like this one for Nebula Graph should be added in the future. If you think the contributing section is not needed now, just remove it.

If you think the current requirements setup is good enough, feel free to close this PR.

This is the last improvement for nebula-python in my todo-list. Thank you and bye!

[Aiee]

Hi, @greyli
Thank you again for bringing these good practices to Nebula python. Looking forward see you around!