[Documentation]: FormRenderProps type also has a values prop

While struggling with an issue to get access to the value of peer fields on a form (see issue on [Stack Overflow](https://stackoverflow.com/questions/48654942/accessing-the-value-of-sibling-fields-on-a-react-final-form-component)), I found resolution in FormRenderProps.values, however this is not listed in the documentation at [react-final-form on GitHub](https://github.com/final-form/react-final-form#formrenderprops).

A suggestion to add it... and might be a good idea to add an example to the page such as "Accessing the values of other fields on your form". I created a [sandbox example](https://codesandbox.io/s/2zz168xox0) - feel free to use it.

Open the linked react-final-form documentation at the FormRenderProps section and compare the documented props with the available values prop. Review the linked CodeSandbox example for the intended use case, then update the documentation to list values and add an example showing access to other fields' values.

Written by the indexing model from the issue text.