Add a new Guide: Unit Testing

Create a new guide in the docs/_guides folder, using the same format as **_template.md**

Title: Unit Testing
Content: 

Add front matter to this guide, something like this:

```
---
title: Unit Testing
layout: default
category: Pipelines
summary: Add a short description of the guide here.
date: YYYY-MM-DD
---
```

Add further content here
