Explain how to test a Drake canary

[jwnimmer-tri]

Currently, our examples are pinned run against the latest stable version of Drake.

During development, it's sometimes helpful to run against a WIP build of Drake instead, to see the effect of Drake changes.

We should make it as easy as practical to do that, and document how to do it.

[jwnimmer-tri]

Roughly, I believe it would go like:

• Open a Drake PR.
• Trigger an expermental Wheel build job on the PR (see https://drake.mit.edu/jenkins.html).
• Find the wheel's URL in the log of the Wheel build. (There will be multiple wheels build for different Python versions. Find the wheel that matches the Python version used for this code at the time.)
• Change examples/requirements.in to replace drake with the experimental wheel's URL.
• Re-run ./bazel run //examples:requirements.update -- --upgrade to switch the lockfile to the experimental wheel.