Add 'Documenting Code' to contribution policy

[hackartisan]

Thanks for the suggestions, @mjgiarlo, these have been incorporated. Not sure what the next step is -- should this language be approved on a tech call before merging?

[awead]

@hackmastera Thanks for doing this! :clap: I'd post a note to hydra-tech and give folks a week to comment or raise objections. After that, one of us can just merge it. I'm not against talking it over on a tech call, but we've already done that and I think most of us are on-board with the changes.

[mjgiarlo]

:+1: to @awead's remarks

[dchandekstark]

:+1: :clap: