Add CONTRIBUTING.md

[bambooforest]

@drammock -- what say you we take all these explanations about adding inventories from our email exchanges and create a contributing doc to point people to?

[drammock]

let's discuss this at our next meeting. So far we've done a lot of oversight / QC and still miss some things. That level of oversight is not free; I don't want to encourage contributions from the wider community only to have their PRs languish because we don't have time to rigorously review them. Having a more robust test suite might change my mind... might also be a reason to switch to new inventories coming in as JSON (because easier to validate automatically; saves us some QC work).

[bambooforest]

We agreed we'd skip opening contributions to the public.