Buck: Document how to create test coverage report

Upstream Buck added support for JaCoCo report generation recently.

Document how to invoke test code coverage so that Gerrit devs can
enjoy the reports [1].

[1] http://imgur.com/wSMxIZa

Change-Id: If80bca30d7ba036c15ab3504855d4b0a6f6abc3c
1 file changed