Auto merge of #5578 - cardoe:extend-ci, r=alexcrichton
authorbors <bors@rust-lang.org>
Sun, 27 May 2018 22:52:20 +0000 (22:52 +0000)
committerbors <bors@rust-lang.org>
Sun, 27 May 2018 22:52:20 +0000 (22:52 +0000)
add GitLab CI to the CI docs

Cargo supports GitLab CI natively for CI badges so include a basic
example of how to use it in the CI section of the documentation.


Trivial merge