docs: Add Unbug to the list of related crates

[BrainBacon]

Motivation

As suggested on Hacker News. Unbug optionally uses Tracing's error! macro in the ensure! and fail! macros.

Solution

Updated README.md and lib.rs to include a Github link.

[BrainBacon]

As a side note, I'm not sure if there is something set up to generate README.md from the documentation in lib.rs, but they seem out-of-sync. I didn't change that here so this PR stays related to just the Unbug change.