Add Usage and Table of Contents, to the README file, enhance Installation section, and fix top header

[NicholasLindner]

Improves README file with new additions and fixes.

Overview:

1. Adds a Table of Contents section for easy navigation and readability.
2. Adds a Usage section to provide users with a better understanding of how to use PyMC-Bart. This will help them get started and on their way.
3. Improves the Installation section by providing descriptions of why each type of installation process might be preferred for a specific user. This will especially help less experienced users.
4. Fixed the title header by capitalizing "Programming".

[aloctavodia]

Thanks @NicholasLindner