Fail publish if ^master is found in README.md (#1378)

justfile

@@ -72,8 +72,9 @@ publish:
   set -euxo pipefail
   rm -rf tmp/release
   git clone git@github.com:casey/just.git tmp/release
-  VERSION=`sed -En 's/version[[:space:]]*=[[:space:]]*"([^"]+)"/\1/p' Cargo.toml | head -1`
   cd tmp/release
+  ! grep '<sup>master</sup>' README.md
+  VERSION=`sed -En 's/version[[:space:]]*=[[:space:]]*"([^"]+)"/\1/p' Cargo.toml | head -1`
   git tag -a $VERSION -m "Release $VERSION"
   git push origin $VERSION
   cargo publish