Add how to contribute guidance

[kelliedesigner]

Summary

Add guidance on how to contribute to the Pattern Library.

Motivation

To encourage contribution, and reduce the need for manual support.

Working doc

https://docs.google.com/document/d/1uNEbfd7tH5Ma6kCX5PoRLs4HW6yfza80m8X_0f_BcRw/edit?usp=sharing