Auto merge of #4237 - vignesh-sankaran:blacklist-url-doc, r=carols10cents
Auto merge of #4237 - vignesh-sankaran:blacklist-url-doc, r=carols10cents

Added documentation blacklisting notes

Added notes regarding [documentation domain blacklisting](https://github.com/rust-lang/crates.io/issues/626) on crates.io

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