Auto merge of #3597 - Susurrus:badge_docs, r=alexcrichton
Auto merge of #3597 - Susurrus:badge_docs, r=alexcrichton

Add gitlab to the supported services for AppVeyor badges.

This is undocumented but supported behavior for AppVeyor. I have already done this with a crate I own and the badge works on both the AppVeyor and crates.io end.

rustyhorde/cargo,rustyhorde/cargo,rust-lang/cargo,nipunn1313/cargo,nipunn1313/cargo,vojtechkral/cargo,nipunn1313/cargo,eminence/cargo,vojtechkral/cargo,eminence/cargo,rust-lang/cargo,vojtechkral/cargo,tshepang/cargo,durka/cargo,tshepang/cargo,tsurai/cargo,achanda/cargo,sbeckeriv/cargo,sbeckeriv/cargo,tshepang/cargo,vojtechkral/cargo,durka/cargo,durka/cargo,tsurai/cargo,achanda/cargo,durka/cargo,nipunn1313/cargo,sbeckeriv/cargo,achanda/cargo,rustyhorde/cargo,sbeckeriv/cargo,tsurai/cargo,achanda/cargo,eminence/cargo,rust-lang/cargo,rustyhorde/cargo,tsurai/cargo,eminence/cargo,rust-lang/cargo,tsurai/cargo,tshepang/cargo