Add documentation "Rewrite URLs"

It's possible to modify URLs before sending them to Ackee. This allows you to…

…remove private data from URLs …remove parameters you don't care about …add a path when your navigation state is managed by your app (React) instead of being visible in the URL

It would be cool if someone could write a documentation about how to modify URLs. It's not 100% Ackee related as modifying URLs happens outside of ackee-tracker.

Here's an example:

1. Get default attributes

const attributes = ackeeTracker.attributes()
// or
const attributes = ackeeTracker.attributes(true)

2. Adjust the attributes

Ackee uses window.location.href for the siteLocation. You can simply replace this value and provide a custom URL. This allows you to remove unnecessary parameters. You can also transform a state-based navigation into a readable URL.

const customAttributes = {
	...attributes,
	siteLocation: removeParameters(window.location.href),
}

3. Create an instance and track the visit

const instance = ackeeTracker.create({
	server: 'https://example.com',
	domainId: 'hd11f820-68a1-11e6-8047-79c0c2d9bce0'
})

instance.record(customAttributes)

This is related to #76 and #113