Create CONTRIBUTING.md

[yuchenlintt]

Since we're planning to expand the team, I thought I'd draft a CONTRIBUTING.md to codify developer practices and hopefully avoid some redundant questions as new folks join. Let me know what you think!

Some proposals:

1. What do you think of letting non-@hal909 team members review obvious and minor PRs? This would let us fix merge conflict CircleCI breakages, add documentation, fix typos, etc. without being blocked on Hal. But all dev feature development, refactoring, contract logic changes, etc. would still need Hal's approval.

2. Right now we've got 77 branches. Could we make it okay to delete merged and stale (>1 month) branches? If people want to back up a branch, they can do so locally or onto a personal fork.

3. I think we should encourage generating docs and flattened contracts as much as possible, without slowing down dev work by requiring these as we do with CircleCI checks. Flattened files falling out of sync introduces room for deploy error.

4. Deploying to Ropsten should also be as frictionless as possible, to speed up testing and reduce deploy errors because we frequently stress test it. Ideally something like yarn deploy --ropsten FooContract, which:

• signs + sends the transaction
• updates the repo's copy of the contract address
• calls the Etherscan API to verify the flattened contract
  All we would have to do is commit and submit a PR. Later we might also upgrade by script:
• transfer Ropsten proxy ownership
• initialize contracts
• update all descendants of the contract