add pre-commit hook

.githooks/pre-commit

@@ -0,0 +1,8 @@
+#!/bin/sh
+git diff --quiet --cached -- scripts && exit 0
+git diff --name-only -- scripts | grep -F "$(git diff --cached --name-only -- scripts)" && exit 1
+version="$(date -u +%Y.%m.%d.%H%M%S)"
+git diff --cached --name-only -- scripts | while read -r file; do
+	sed -i "s/@version .*/@version $version/" "$file"
+	git add "$file"
+done

README.md

@@ -5,6 +5,16 @@
 * [TV Tropes thing](https://chor.date/tvtropes.user.js) - A script to remove the "This is page #x you have viewed this month without ads" banner.
 * [YouTube thing](https://chor.date/youtube.user.js) - A script to unset autoplay and display hidden video tags.
 
+## Development
+
+This includes a pre-commit Git hook to automatically update the `@version` tags on the userscripts. To enable it, point Git to use the hooks from the `.githooks` directory.
+
+```
+git config core.hookspath .githooks
+```
+
+This hook script will prevent the commit if there are any files in the `scripts` directory with both staged and unstaged changes.
+
 ## License
 
 [MIT](LICENSE)