Update CONTRIBUTING.md with Poetry shell install (#458)

## Problem
Poetry shell is now a plugin that should be installed separately.

## Solution

## Type of Change

- [ ] Bug fix (non-breaking change which fixes an issue)
- [ ] New feature (non-breaking change which adds functionality)
- [ ] Breaking change (fix or feature that would cause existing
functionality to not work as expected)
- [ ] This change requires a documentation update
- [ ] Infrastructure change (CI configs, etc)
- [x] Non-code change (docs, etc)
- [ ] None of the above: (explain here)

## Test Plan

Describe specific steps for validating this change.

Author: avi1mizrahi

CONTRIBUTING.md

@@ -44,6 +44,7 @@ Change directory into the repository, as we'll be setting up a virtualenv from w
 ### Step 1. Install Poetry
 
 Visit [the Poetry site](https://python-poetry.org/) for installation instructions.
+To use the [Poetry `shell` command](https://python-poetry.org/docs/cli#shell), install the [`shell` plugin](https://github.com/python-poetry/poetry-plugin-shell).
 
 ### Step 2. Install dependencies