Update contributing guidelines

[yeastplume]

Will give a couple of days for reviews or suggestions before merging

[lehnberg]

Does it make sense to also include specific instructions to update any documentation that is affected by the PR in question?

[JeremyRubin]

I think that you could make pull_request_template.md with sections for:

* Includes a proper description of what problems the PR addresses, as well as a detailed explanation as to what it changes
* Explains whether/how the change is consensus breaking or breaks existing client functionality
* Contains unit tests exercising new/changed functionality
* Fully considers the potential impact of the change on other parts of the system
* Describes how you've tested the change (e.g. against Floonet, etc)

which might be a bit friendlier for first time contributors.

[ignopeverell]

I think @JeremyRubin PR template makes sense as well.