README: demonstrate a weekly flake.lock update

It's arguably not that useful to be inundated with flake.lock update PRs
every day, so do it once a week on Sunday.

README.md

@@ -13,7 +13,7 @@ name: update-flake-lock
 on:
   workflow_dispatch: # allows manual triggering
   schedule:
-    - cron: '0 0 * * *' # runs daily at 00:00
+    - cron: '0 0 * * 0' # runs weekly on Sunday at 00:00
 
 jobs:
   lockfile:
@@ -33,3 +33,15 @@ jobs:
         uses: DeterminateSystems/update-flake-lock@v3
 ```
 
+## Running GitHub Actions CI
+
+GitHub Actions will not run workflows when a branch is pushed by or a PR is opened by a GitHub Action. To work around this, try:
+
+```
+git branch -D update_flake_lock_action
+git fetch origin
+git checkout update_flake_lock_action
+git commit --amend --no-edit
+git push origin update_flake_lock_action --force
+```
+

action.yml

@@ -30,3 +30,17 @@ runs:
           ```
           ${{ steps.commit_message.outputs.msg }}
           ```
+
+          ### Running GitHub Actions on this PR
+
+          GitHub Actions will not run workflows on pull requests which are opened by a GitHub Action.
+
+          To run GitHub Actions workflows on this PR, run:
+
+          ```sh
+          git branch -D update_flake_lock_action
+          git fetch origin
+          git checkout update_flake_lock_action
+          git commit --amend --no-edit
+          git push origin update_flake_lock_action --force
+          ```