Auto merge of #1744 - sgrif:sg-allow-headers, r=bryanburgers
Auto merge of #1744 - sgrif:sg-allow-headers, r=bryanburgers

Allow h4-h6 tags in READMEs

Fixes #1717

rust-lang/crates.io,rust-lang/crates.io,rust-lang/crates.io,rust-lang/crates.io