Add core lib readme and workflow to auto-close PRs (#11354)

* Add GitHub workflow to close PRs on core-lib repo

* create readme for core-lib folder

* Update app/README.md

Co-authored-by: John Linhart <[email protected]>

Co-authored-by: John Linhart <[email protected]>

Author: RCheesley

.github/workflows/close_pull_requests.yml

@@ -0,0 +1,26 @@
+# Workflow name:
+name: Close Pull Requests
+
+# Workflow triggers:
+on:
+  pull_request_target:
+    types: [opened]
+
+# Workflow jobs:
+jobs:
+  run:
+    runs-on: ubuntu-latest
+    steps:
+    - uses: superbrothers/close-pull-request@v3
+      with:
+        comment: |
+          Thank you for submitting a pull request. :raised_hands:
+          
+          We greatly appreciate your willingness to submit a contribution. However, we are not accepting pull requests against this repository, as all development happens on the [main project repository](https://github.com/mautic/mautic).
+          
+          We kindly request that you submit this pull request against the [respective directory](https://github.com/mautic/mautic/blob/head/app/) of the main repository where we'll review and provide feedback. If this is your first Mautic contribution, be sure to read the [contributing guide](https://github.com/mautic/mautic/blob/head/.github/CONTRIBUTING.md) which provides guidelines and instructions for submitting contributions.
+          
+          Thank you again, and we look forward to receiving your contribution! :smiley:
+          
+          Best,
+          The Mautic team 

README.md

@@ -0,0 +1,5 @@
+# Mautic Core-lib repo
+
+## This is a mirror repository of the /app folder of Mautic, and is managed centrally in https://github.com/mautic/mautic/blob/head/app.  This is a read-only mirror repository.
+
+**📣 Please make PRs and issues against the [mautic/mautic](https://github.com/mautic/mautic) repository, not here!**