Bitrise workflow details

Here's the deal with Bitrise.

1. As of f59d4ba (already in master), the config for running through our steps for Bitrise has been moved out of their GUI workflow builder and into bitrise.yml in the repo root (for whatever branch).

2. The Bitrise config for the repo has been set to just run the relevant branch's bitrise.yml.

3. If you want to change the bitrise.yml, do so and use the bitrise CLI tool (brew install bitrise or manual install) to bitrise validate.

4. Change the primary workflow in your branch to see how changes will be run on Bitrise before merging them to master, affecting all branches (since all branches run their respective primary workflow). You can create alternate workflows, named after branches, to have them run by commits in that branch, but generally we will just use primary.

5. You can also create custom non-branch-related workflows in the bitrise.yml beyond the default primary one, such as eventually where we will have a roll_build or similar workflow to address #2844.

6. Both [skip ci] and [ci skip] work for our Bitrise setup for this repo as of f5bb746.

More on Bitrise's steps library (for example, we use the select-xcode-version, script, xcode-test, and slack steps): https://github.com/bitrise-io?utf8=✓&query=steps and http://www.steplib.com