Auto merge of #3087 - carols10cents:test-instructions-in-readme, r=alexcrichton
Auto merge of #3087 - carols10cents:test-instructions-in-readme, r=alexcrichton

Document recommended way for running tests

Fixes #3084.

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