flake-update: init action

README.md

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+# update-flake-lock
+
+This is a GitHub Action that will update your flake.lock file whenever it is run.
+
+> NOTE: We hardcode the `install_url` to a relatively recent `nixUnstable` (`nix-2.5pre20211015_130284b` currently). If you need a newer version that includes a new feature or important bug fix, feel free to file an issue or send a PR bumping the `install_url` inside the [`action.yml`](action.yml)!
+
+## Example
+
+An example GitHub Action workflow using this action would look like the following:
+
+```yaml
+name: update-flake-lock
+on:
+  workflow_dispatch:
+
+jobs:
+  lockfile:
+    runs-on: ubuntu-latest
+    steps:
+      - name: Checkout repository
+        uses: actions/checkout@v2
+      - name: Update flake.lock
+        uses: DeterminateSystems/update-flake-lock@main
+```
+
+To have this workflow run on a recurring basis, see the [GitHub Actions documentation on the `schedule` key](https://docs.github.com/en/actions/learn-github-actions/events-that-trigger-workflows#schedule).