Readme `inputSourceMap` mention

[luwes]

inputSourceMap is now default true in Babel.
https://babeljs.io/docs/en/options#inputsourcemap

crazy enough I have a project importing sinuous where this Babel option inputSourceMap has to be set to false and add this plugin for sourcemaps to work properly.

might need to be reflected in the readme.

[edsrzf]

I also have a build where setting inputSourceMap to false, combined with this plugin, makes my source maps work correctly. Thank you for mentioning it in this issue, as I don't think I would have thought to try that otherwise.