Add API documentation to README

[GuiltyDolphin]

Need to add information on the following:

• What a monitor is.
• What monitor instances are.
• define-monitor format and conventions.
• monitor format and conventions.
• Existing monitors and their use and attributes.

[GuiltyDolphin]

It might be worth doing an overview in the README, with more comprehensive documentation in a monitor.org file that can compile to texinfo, which would serve as much better documentation and reference within Emacs.