Add comments to the starter filters.js file

[BenSurgisonGDS]

What

Add comments to the starter filters.js file to guide users to the appropriate documentation

Why

A returning designer was unaware that the format of creating filters had changed since moving to version 13.

Who

Developer

Done when

The contents of the prototype starter filters.js file is as follows:

//
// For guidance on how to create filters see:
// https://prototype-kit.service.gov.uk/docs/filters
//

const govukPrototypeKit = require('govuk-prototype-kit')
const addFilter = govukPrototypeKit.views.addFilter

// Add your filters here