Update how-to.md with a Layout and View Example

[appelgran]

I have made another example for the how-to document. I missed this information when using Stubble for the first time this week. I had to look into mustache.php's documentation to grasp this. Fortunately I also know PHP, but not everyone does.

Feel free to make changes or place it elsewhere.

[Romanx]

Hi there,

Thanks for the contribution, I think this example may be a little explict for our use case rather than how to use partial templates. Maybe we'll add a recipes folder to put things like this that are standard mustache but the Stubble syntax for them. I'm going to close this for now an open an issue for it under #109