Bundler 2.0 was released recently, but none of our lockfiles are
associated with this version — they all refer to Bundler 1.x.
Unfortunately, Travis is now attempting to use Bundler 2.0 by default,
so we have to ensure that it is using the correct version as well.
* Move section on documentation from README to CONTRIBUTING
* Add a setup script that contributors and maintainers can use to set up
a dev environment
[skip ci]