[Documentation] Add one markdown file (.md) showing (1) the API of Readline and (2) the API of Reline

Would it be possible to add one .md markdown file,
that is like a table layout?

On the left side, there should be the old Readline API.

On the right side, there should be the new Reline API.

Ideally both should be up-to-date.

This file could be put under https://github.com/ruby/reline/tree/master/doc
or doc/reline there. The name could be "API-table" or "API_comparison_table.md"
or something like that.

It does not have to be complete; it can be kept as a partial helper, just showing
the more important API changes (if any).

For instance:

Reline.readmultiline

Specifically, that .md file could also include some helpful advice, and also notices
which methods haven't been ported yet, or which may behave differently.

At any rate, thanks for reading this issue request. I am currently porting some
old Readline code, over to Reline, and the documentation can ideally be improved
for Reline, so that we poor souls can all abandon Readline eventually.

Start by reviewing the existing doc/reline directory and the current Readline and Reline APIs. Create one Markdown comparison table covering the important methods, including readmultiline, and note methods that are unported or behave differently. Done means the file is an up-to-date, useful partial migration reference.