[docs] developing with a local version of libhoney in the beeline's example application

[emilyashley]

Which problem is this PR solving?

• When developing changes for beeline and libhoney concurrently, I wanted to run both at the same time locally in my development environment (rather than relying on published versions of libhoney). This provides documentation on how I did that :)

Short description of the changes

• reduce complexity of npm script in example app
• add documentation for local development

[robbkidd]

👍🏻 Works on my machine, which in this case is the point.

I added the suggestion for changing directory in a subshell with (cd ../.. && … ) below, but also I found that npm run setup had me still in the examples/node-tracing/ directory when it completed. Including the parens doesn't appear to change the behavior, but would only prevent people like me from suggesting seems-like-a-good-idea subshell parens in the future. 😁