diff --git a/README.md b/README.md
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+
+# platform manifest for postmarketOS's anbox image
+
+have a look at https://gitlab.com/postmarketOS/anbox-image-make/
+for the build instructions.
+
+please don't rely on the commit hash in this repo, as they will be
+rebased.
+
+## `pin-this-tree.sh`
+
+this tool is used to maintain this repository: it pins every git tree
+to its currently checked out commit.
+
+How to pin the commits:
+
+1. in this repo, get back to the original, non-pinned,
+   version of the manifest
+```sh
+git checkout unpinned default.xml
+```
+
+2. get the android source
+```sh
+cd anbox-image-make # (that's the git repo above)
+make fetch && date=$(date -u +%s) # get the latest version of the source
+```
+
+3. pin the versions and create the tarball
+```sh
+cd source # anbox-image-make/source, or wherever you did put it
+repo forall -c '~/platform_manifests/pin-this-tree.sh'
+tar -czf anbox-image-$date.tar.gz *
+```
+
+then go back to this repository, `git commit default.xml -m "pin to $date"`, `git push` and upload the tarball.
+
+## Note for maintainers
+
+When not pushing a change that pin commits, remember to update the unpinned tag: `git tag -f unpinned`
diff --git a/pin-this-tree.sh b/pin-this-tree.sh
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+#!/bin/sh
+# pin every repo to the current checked out commit
+# this script assumes no commit is already pinned
+
+# tested with GNU and Busybox sed.
+
+if [ -n "$1" ]; then
+	manifest="$1"
+else
+	dir="$(realpath "$(dirname "$0")")"
+	if [ -z "$dir" ]; then
+		dir=.
+	fi
+	manifest="$dir"/default.xml
+fi
+
+if ! [ -f "$manifest" ]; then
+	echo "could not find $manifest. make sure to cd" \
+	     "in the platform_manifest directory that contains it"
+	exit 1
+fi
+
+commit="$(git rev-parse HEAD)"
+path="$REPO_PATH"
+
+sed "$(cat <<EOT
+	# find the line that references this path
+	\| path="$path"|    s| />| revision="$commit" />|
+	                    # … and append the desired commit
+EOT
+)" "$manifest" -i