Improve README on Opening PRs for Adding Streamers

[TheCedarPrince]

Background

We want more streamers added to the site! As of right now, however, they don't know how to add themselves. Let's fix that!

Problem

Detail how to add themselves to the website:

• Note about default icon if they want it
• Step-by-step example
• Improve documentation for streamer icons

[logankilpatrick]

Docs are pretty clear right now but these things above would all be big improvements!