Update CONTRIBUTING.md to clarify repository cloning instructions and add code review SLA (#3504)

Author: tsck

CONTRIBUTING.md

@@ -55,17 +55,15 @@ Before contributing, ensure you have these tools installed:
 
 ## Getting Started
 
-### 1. Fork and Clone
+### 1. Clone the Repository
 
 ```bash
-# Fork the repository on GitHub, then clone your fork
-git clone https://github.com/<your-username>/leafygreen-ui.git
+git clone https://github.com/mongodb/leafygreen-ui.git
 cd leafygreen-ui
-
-# Add the upstream remote
-git remote add upstream https://github.com/mongodb/leafygreen-ui.git
 ```
 
+> **Note**: PRs from forked repositories cannot be merged. Please clone the repository directly and push your branch to the main repo.
+
 ### 2. Install Dependencies
 
 ```bash
@@ -334,6 +332,8 @@ git push origin <your-branch-name>
 
 The team will review your PR and may request changes.
 
+> **Code Review SLA**: We aim to provide initial review feedback within 24 hours of PR submission.
+
 ### 6. Squash and Merge
 
 - Once you've received approval from all maintainers who have commented, you can squash and merge your branch.