Documentation on Cache with `babel-register`

[geyang]

@zvictor,

Thanks for this useful plugin!

I recently ran into caching issue as well, and it took quite some search to figure out what is happening: #7 #15

Description of the problem

when used with babel-register as in node -r babel-register index.js, some how nod always load an older version of the markdown file I'm reading. Even after restarting the node process.

PR Suggestion

Can we:

1. make the https://github.com/Quadric/babel-plugin-inline-import#caveats session more prominent in the README?
2. and more importantly, the current explanation in the caveat session doesn't really explain what importing and imported are. I still don't know what is happening and why babel-register has problem with cached file content event after restarting the node process. Would you mind explain what is causing the problem above?

Best Regards