Update Getting Started (How to change the project, why?)

@peterflynn and I talked about this on gitter.
Sorry for the delay. Here's a suggested rewording (also note the added heading and the paragraph/samp structure):

```
<h3>Projects in Brackets</h3>
<p>In order to edit your own code using Brackets, you can just open the folder containing your files. Brackets treats the currently open folder as a "project"; features like code hints, Live Preview and Quick Edit only use files within the currently open folder.</p>

<samp>Once you're ready to get out of this sample project and edit your own code, you can use the dropdown in the sidebar to switch folders. Right now, the dropdown says "Getting Started" - that's the folder containing the file you're looking at right now. Click on the dropdown and choose "Open Folder..." to open your own folder. You can also use the dropdown later to switch back to folders you've opened previously, including this sample project.</samp>
```