docs for post commit hook with dirmd

docs/ADDING_DIRMD_TO_GIT_POST_COMMIT_HOOKS.md

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+# Automating sending files to proton drive on commits
+
+If you add this to any repos, and have dirmd in path and logged in on proton drive cli, then after each commit the newest code is pushed to proton drive, as vertical slices. If you do not want vertical slices of the code omit --vertical-slices. Also check out the other flags of dirmd if required.
+
+## repos-root/.git/hooks/post-commit 
+
+create this file
+
+```sh
+#!/bin/bash
+
+set -euo pipefail
+
+if ! command -v dirmd &>/dev/null; then
+    exit 0
+fi
+
+REPO_ROOT=$(git rev-parse --show-toplevel)
+REPO_NAME=$(basename "$REPO_ROOT")
+
+dirmd -o "/tmp/${REPO_NAME}_new" "$REPO_ROOT" -f --vertical-slices --proton-drive
+```
+
+and then make it executable with something like `chmod +x .git/hooks/post-commit`