Note quotation marks for installation (#466)

## Description

<!-- Provide a brief description of the PR's purpose here. -->

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](https://github.com/icaros-usc/pyribs/blob/master/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

README.md

@@ -100,6 +100,7 @@ pyribs supports Python 3.8 and above. The vast majority of users can install
 pyribs by running:
 
 ```bash
+# If you are on Mac, you may need to use quotations, e.g., pip install "ribs[visualize]"
 pip install ribs[visualize]
 ```