Doc, CONTRIBUTING.md has incorrect `pip install -e ...` command at `Step 6` in `Your first code contribution` section

[john-liu2]

Doc, CONTRIBUTING.md has incorrect pip install -e ... command at Step 6 in Your first code contribution section

The bad one:

$ pip install -e .[dev]    
zsh: no matches found: .[dev]

$ python -m pip install -e .[dev]
zsh: no matches found: .[dev]

The good one:

$ pip install -e '.[dev]'  
Looking in indexes: https://pypi.python.org/simple
...

$ python -m pip install -e '.[dev]'
Looking in indexes: https://pypi.python.org/simple
...