Add contribution guidance to the readme

[pfwd]

This may seem pretty obvious to most but at first I wasn't I wasn't sure if proposals were being accepted via GitHub Issues or pull requests.
I think the readme needs a section on how to submit a proposal with a brief explanation on the acceptance/rejection process.

[daveajones]

Good call. I may have an even better solution shortly. Going to talk about it on the board meeting tomorrow.

[pfwd]

@daveajones Great, I'm glad it's being brought up. It might be worth having a PR or issue template for proposals so a common submission format can be followed.