Add an overview of libcnb features to the README

[edmorley]

A suggestion was made on Slack (here) that the libcnb README/docs include a top level feature list with links to specific examples & docs.rs

eg:

Features

• build
• detect
• creating cached & uncached layers
• error handling
• managing composite buildpacks
• inter-buildpack configuration with build plans
• embedded binaries for exec.d & process types