A little problem with the README.md file

[tjcchen]

Default option should also leave exclude as null, just like the source code did.

I cannot initiate a PR on this repo, can you please help fix it. Thank you!

{
    rootValue: 16,
    unitPrecision: 5,
    propList: ['font', 'font-size', 'line-height', 'letter-spacing'],
    selectorBlackList: [],
    replace: true,
    mediaQuery: false,
    minPixelValue: 0,
    exclude: null
}