Adding section on function expressions

Added a new section detailing the use of function expressions and why.

Generally, I think it will add to code readability to define functions as expressions rather than using function declarations. This will allow us to define our functions as the top of a code block, providing some predictability when others are reading our code -- variables are declared at the top, invocations etc below.

Status: **Ready to merge**

Reviewers: @donnielrt @tedtate @jansepar @noahadams 

## Changes
- added section on function expressions
- added proper headers for readability

## How to test-drive this PR
- read the changes