Added Git Hooks

.github/hooks/README.md

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+# Git Hooks
+These work even on Windows because of Git Bash.
+
+## Setup
+`git config core.hooksPath .github/hooks`

.github/hooks/post-checkout

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+#!/bin/sh
+git submodule update --init --recursive

.github/hooks/post-merge

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+#!/bin/sh
+git submodule update --init --recursive

.github/hooks/pre-push

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+#!/bin/sh
+if git diff --cached --submodule | grep -q "^+"; then
+  echo "WARNING: You have unpushed changes in submodules."
+  exit 1
+fi