Update docs to show how to customize the form

[stephtaylor]

See #85

The current docs don't show any examples for how to style the input fields. There are a few references to the ability to do so, but nothing showing exactly what you can change.

Ex: https://developer.squareup.com/docs/api/paymentform#datatype-inputstyleobjects

[raptoria]

It would also be really handy to have typescript declarations all around. I was scratching my head for quite a while when I added a style to inputStyles that wasn't "allowed" and it failed silently (if you add one non-allowed style, all the styles fail).

[znwhite]

For anyone viewing this issue looking for help - don't import the css file the way the package tells you to:

import 'react-square-payment-form/lib/default.css'

Instead, just copy and paste the css file directly into your codebase like a true gangster and import that file instead. You then have all the styles locally and can edit them to update the styling of the form.

[jeevanpillay0000]

For anyone viewing this issue looking for help - don't import the css file the way the package tells you to:

import 'react-square-payment-form/lib/default.css'

Instead, just copy and paste the css file directly into your codebase like a true gangster and import that file instead. You then have all the styles locally and can edit them to update the styling of the form.

This x100. Thank you :)