Auto merge of #4464 - matklad:document-all-the-things, r=alexcrichton
Auto merge of #4464 - matklad:document-all-the-things, r=alexcrichton

Mention sccache in the guide

Closes  #4307

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