adds githook for lastmod line

.githooks/pre-commit

@@ -0,0 +1,11 @@
+#!/bin/sh
+# Contents of .git/hooks/pre-commit
+# Replace `last_modified_at` timestamp with current time
+# https://mademistakes.com/notes/adding-last-modified-timestamps-with-git/
+
+git diff --cached --name-status | egrep -i "^(A|M).*\.(md)$" | while read a b; do
+  cat $b | sed "/---.*/,/---.*/s/^lastmod =.*$/lastmod = \'$(date -u "+%Y-%m-%dT%H:%M:%S%z")\'/" > tmp
+  mv tmp $b
+  git add $b
+done
+