Add reminder for release day

Author: andyw8

.github/api_update_reminder_on_release.md

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+---
+title: A new release of the Shopify API occurred
+assignees: @platform/dev-tools-education
+labels: automated
+---
+
+This is an automated reminder for the maintainers that a new Stable release of the Shopify API was scheduled for today,
+so a new release of the library is now due.
+
+A draft PR should already exist for this.
+
+Review the changelog again and consider if anything in the library needs to change:
+
+https://shopify.dev/changelog
+
+Test against the new release by using the Stable version just releaesd
+
+Aim to release an update of the library within one week.
+
+Thank you!

.github/workflows/api_update_reminder_on_release.yml

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+on:
+  schedule:
+    - cron: "0 0 1 1,4,7,10 *" # At 00:00 on 1st of January, April, July, and October
+
+name: API update reminder
+jobs:
+  reminder:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v2
+      - uses: JasonEtco/[email protected]
+        env:
+          GITHUB_TOKEN: ${{ secrets.GITHUB_TOKEN }}
+        with:
+          filename: .github/api_update_reminder_on_release.md