babel-preset-react-union informations

Please add to README.md, some information for **babel-preset-react-union**. For me wasn't intuitive to instantly look into `/packages`. From **.babelrc** file it's to much magic behind it.
And also https://www.npmjs.com/package/babel-preset-react-union doesn't contain any description (README).

Start by reading README.md and the .babelrc file, then inspect the babel-preset-react-union package under /packages to understand what information is missing. Done means the README explains the package clearly enough to find and understand it without relying on the .babelrc, and the npm package has a description.

Written by the indexing model from the issue text.