update contributing to resolve issue #321

[JuLeeAtPlanet]

Closes #321 Add zsh command for pip install in CONTRIBUTING.md #321

Related Issue(s):

Closes #321

Proposed Changes:

For inclusion in changelog (if applicable):

1. N/A

Not intended for changelog:

1. pip install -e .'[dev]' needs quotes

Diff of User Interface

Old behavior:
pip install -e .[dev] prevents install on zsh
New behavior:
pip install -e .'[dev]' ensures install on zsh

PR Checklist:

• This PR is as small and focused as possible
• [] If this PR includes proposed changes for inclusion in the changelog, the title of this PR summarizes those changes and is ready for inclusion in the Changelog.
• [] I have updated docstrings for function changes and docs in the 'docs' folder for user interface / behavior changes
• This PR does not break any examples or I have updated them

(Optional) @mentions for Notifications: