fix(mapsApiCallback): added declare of callback in README

[theophanemayaud]

Summary Added declare of callback before api load in README, for when PlacesAutocomplete is not mounted on first load/land page, because it might cause a load error from google, and then PlacesAutocomplete will not have maps places api loaded. Fixes #225

Before submitting a pull request, please make sure the following is done...

1. Fork the repo and create your branch from master. OK
2. If you've added code that should be tested, add tests! NA
3. If you've changed APIs, update the documentation. Just docs change
4. Ensure that:

• The test suite passes (npm test) OK although not necessary
• Write formatted git commits by using npm run commit Didn't manage to make it work... got a error : TypeError: require.extensions.hasOwnProperty is not a function at requireDir (/Users/theophanemayaud/Internet/react-places-autocomplete/node_modules/require-dir/index.js:93:37)

[theophanemayaud]

Wait, I'm still getting some other errors... And also, sometimes the input is disabled. I have to do some more checks.

[theophanemayaud]

Well, now it just works for me !