add helper script to tag release

contrib/tag-release.sh

@@ -0,0 +1,13 @@
+c_awk_latest='
+/^## \[.*\]/ {if (latest) exit; else {latest=1; next}}
+latest {print}
+'
+
+tag=$(grep ^version Cargo.toml | cut -d" " -f3 | tr -d '"')
+changes=$(awk "$c_awk_latest" CHANGELOG.md | tail -n+2)
+
+git -c core.commentchar='%' tag --edit \
+    -m "Release $tag" \
+    -m "$changes" \
+    "$tag"
+git push github master "$tag"