Document 'google-cloud-trace' and BackgroundThreadTransport

[stewartreichling]

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[stewartreichling]

I signed it!

[googlebot]

CLAs look good, thanks!

[liyanhui1228]

This pip and pipenv target are defined at the beginning, please fix this in order to make the CI green.

nox > python setup.py check --restructuredtext --strict
running check
warning: check: Duplicate explicit target name: "pip".

warning: check: Duplicate explicit target name: "pipenv".

error: Please correct your package.

[odeke-em]

Thank you @stewblr for the fix and @liyanhui1228 for the review and merge!

@stewblr in the future, in your commit message, please add the phrase Fixes #<issueNumber> so that Github can auto-close the issue, for example this one could be

README: update Stackdriver*Exporter pip instal and need for BackgroundThreadTransport 

Fixes #260

and that would have auto-closed this issue after the PR had been merged :)

[stewartreichling]

Will do, thanks for the tip!