Add Prerequisites to Readme (#936)

## Motivation
Users need to know what **prerequisites** they need in order to run exo.
Simple addition to docs prevents future raised issues.

## Changes

Updated ``README.md``:
- to include installation instructions for
**[uv](https://github.com/astral-sh/uv)** and
**[macmon](https://github.com/vladkens/macmon)**.

Updated ``CONTRIBUTING.md``:
-  to verify these prerequisites are met before starting development.

- Standardized on brew installation instructions for macOS users to keep
the guide simple.

## Why It Works

By listing these prerequisites upfront, users will set up their
environment correctly before attempting to run exo.

## Test Plan

### Manual Testing
MacBook Pro M4
- Verified that ``uv`` and ``macmon`` were missing initially, causing
failures
- after installing them via brew (as documented), uv run exo starts
successfully.

### Automated Testing
<!-- Describe changes to automated tests, or how existing tests cover
this change -->
<!-- - -->

---------

Co-authored-by: Evan Quiney <evanev7@gmail.com>

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