The footer is technically part of default.master.php:

<div id="Foot">
<?php
this->RenderAsset('Foot');
?>
</div>

So you could do this 1 of 2 ways: either add your code to the #Foot div, or use a plugin to add things to the Foot asset which then gets rendered on the fly as the page loads. I recommend the latter for flexibility if you want to learn how to do even cooler stuff; the former for expedience if you just wanna get this done and move on.

To do it as a plugin, you'll want a method like this in your hooks file:

public function Base_Render_Before(&$Sender) {
$Sender->AddAsset('Foot', 'Function call or text to add');
}

Search the source code for "AddAsset" to see some examples of its use and the notes on how to use it.