Auto merge of #5497 - dwijnand:patch-1, r=alexcrichton
Auto merge of #5497 - dwijnand:patch-1, r=alexcrichton

Mention +nightly in ARCHITECTURE.md

See the conversation on Gitter: https://gitter.im/rust-lang/cargo?at=5aee83305cf0b830045d0602.

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