Incompatible flag diff script

Sometimes, people flip incompatible flags, but forget to update the GitHub issue. The release manager then has to go through all flags manually and check their real status.

@laurentlb Had the following idea:

> What do you think of a script to diff the incompatible flags between two Bazel binaries?
> If we run "bazel help" and collect the flags, we can print which flags: have been added, removed, and flipped since the previous release. This can be useful for the release manager.

Sounds good. We should then also integrate it into the [release playbook](https://github.com/bazelbuild/continuous-integration/blob/master/docs/release-playbook.md) as a step.

Start by reviewing the proposed bazel help comparison and the release playbook at docs/release-playbook.md. Define the script around comparing incompatible flags from two Bazel binaries and reporting added, removed, and flipped flags. Done means the comparison is usable by release managers and the release playbook includes it as a step.

Written by the indexing model from the issue text.