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](https://github.com/electerious/ackee-tracker#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