Fix readme path on macros/Cargo.toml
macros/ has its own (stub) README, which we want to appear in places where this crate's README exists.
Fixes
warning: readme ../README.md
appears to be a path outside of the package, but there is already a file named README.md
in the root of the package. The archived crate will contain the copy in the root of the package. Update the readme to point to the path relative to the root of the package to remove this warning.