Contributing guidelines

[specious]

I've committed the first version of the contributing guidelines, which GitHub will automatically display to contributors when they go to create a new pull request or open an issue.

Material is selected and adapted from or inspired by various contributor guideline documents that accompany other projects.

What can/should we add or change at this stage to make this document even better?

I've uploaded a google docs version of this document where we can edit it together. Everyone has permission to comment. Please ask me for edit permissions if you don't already have them.

CC: @danieltoben @FragLegs