Add more information in README about span tracing

This PR provides more information about how to get span tracing for Python up and running in Test Analytics.

It includes:

More background into how tracing works in Buildkite Test Analytics collectors generally
Links through to examples in our reference test collector implementation in Ruby
A link directly to the relevant part of code to see SpanCollector documentation

I don't have experience in Python, so if anything here doesn't make sense in a Python context, I would appreciate an edit! 🙇🏻‍♀️

buildkite / test-collector-python