Improve documentation and examples

[briancavalier]

From #178

• Add simple "Introduction" and/or "Why would I use creed" sections
• Add another 1-2 simple examples to the README.
• Add links to helpful information about foundational functional programming concepts, such as the Mostly Adequate Guide.
• More clearly encourage exploration via REPL.
• Emphasize map, et al.
• Group A+ / ES interop functions (i.e. then, resolve) together and explain more clearly that's their purpose. (Move then and resolve to interop section #180)