Document feature differences w.r.t. readline

[ford-prefect]

While it's impressive that linenoise is so much smaller than readline, there are a number of missing features, and it would be good for newcomers to be able to evaluate what they're losing (in addition to what they're gaining). Examples include bash-style line-editing (word-wise navigation, yank support) and file name completion.

[Sonophoto]

You should write that up.

I don't think anyone else cares because we already know what readline does which explains why we use ~1200 lines of linenoise instead of 30K+ lines of readline.