Document custom .babelrc file

[madnificent]

It would be nice if users could override the .babelrc file, probably as experimental behavior.

The .babelrc file specified in the source folder seems to be consumed in the app so far. We should test and document whether this works and indicate whether this is considered supported behavior, how we expect users to cope with such extension, and how we want this to behave in the future.

This could be a solution to have custom ignore files by #27, if not for babel/babel#11394.