Note quotation marks for installation

[btjanaka]

Description

Mac users may need to put quotation marks around their installation commands due to zsh interpreting the square brackets differently. This PR adds a note on that in the installation command.

We also have installation commands in other files like the examples, but I think adding the note here should cover most people. At any rate, we have not yet received any issues about this, so people seem to be figuring it out.

I also updated this on the website.

Status

• [x] I have read the guidelines in CONTRIBUTING.md
• [x] I have formatted my code using yapf
• [x] I have tested my code by running pytest
• [x] I have linted my code with pylint
• [N/A] I have added a one-line description of my change to the changelog in HISTORY.md
• [x] This PR is ready to go