* Problem: Source files contain no license info
Solution: Add comments with SPDX license info to source files
* Python 3 files don't need # -*- coding: utf-8 -*-
Solution: Modify the file `HOW_TO_HANDLE_PULL_REQUESTS.md` to point its readers to the new document about how to handle external pull requests: [BEP-16](https://github.com/bigchaindb/BEPs/tree/master/16).
When someone outside of BigchainDB GmbH submits a pull request, we have a standard comment that we post, asking them to agree to our CLA. @gmcmullen recently wrote [a comment to explain why](https://github.com/bigchaindb/bigchaindb/pull/695#issuecomment-254747113), and I thought it would be a good idea to include that every time we ask.