Non-Github contribution guidelines need clarification

Originating from a conversation with @mxmason.

Currently, our only language around how people who don't use Github can contribute is listed on our Write for us page. There are two issues with this from my perspective:

1. The content is not on our Contributing Guidelines, or cross-linked there.
2. The process is still too opaque to give a potential contributor an idea of what a non-Github content contribution process looks like.

We need to expand on this so it is more transparent and approachable. While we utilize Github as a method to run the site, it should not be a barrier for participation.

Ideally, part of this involves people who have contributed without using Github who would be willing to identify areas of confusion or friction.