Add a rationale to the readme

[akirak]

The goal of this project is to provide a framework with which the user can lock and update individual packages in his/her config. The primary reason behind this goal is security, and the issue has been discussed several times:

• Why do we trust package.el?
• Are emacs package repositories a security risk?
• Borg vs Straight.el?

(There are more reddit threads on the topic, and you can easily reach them via DuckDuckGo.)

I haven't described it in the README yet. I find it difficult to concisely describe the reason, but I will have to do that.

cbowdon provides the following checklist, and Twist would help with the first criteria:

• reviewing every single update prior to installing (hardly feasible)
• code signing (not sure if package.el supports this though)
• waiting before updating, in hopes that an attack would be discovered before you install
• installing fewer packages to limit potential exposure
• generic measures like not running as root and having endpoint protection on Windows

[akirak]

I have briefly explained the intension in README of twist.el. A comprehensive introduction material will be added in the future.