Document new autoscript behavior more fully

Can you explain more fully in the readme.md what the automatic script inclusion mechanism is, why it's useful, and if there are cases where it might fail?

Also can you document it on [NPM](https://www.npmjs.com/package/@ampproject/toolbox-optimizer)?

/cc @garanj . If amp-toolbox becomes the preferred tool here, we should mention that and link to it on [the npm page for amphtml-autoscript](https://www.npmjs.com/package/amphtml-autoscript).

Start with readme.md and review the automatic script inclusion mechanism described by the issue. Update the README and the relevant NPM pages to explain how it works, why it is useful, possible failure cases, and whether amp-toolbox should be linked as the preferred tool.

Written by the indexing model from the issue text.