chore: greet new PR contributors using mergeable

.github/mergeable.yml

@@ -32,3 +32,40 @@ mergeable:
 
             Sincerely, the mergeable bot 🤖
       - do: close
+
+  - when: pull_request.opened, pull_request.ready_for_review
+    filter:
+      - do: payload
+        pull_request:
+          draft:
+            match: false
+          author_association:
+            must_include:
+              regex: ^NONE|FIRST_TIME_CONTRIBUTOR|FIRST_TIMER$
+    validate: []
+    pass:
+      - do: comment
+        payload:
+          body: |
+            Hi there, @@author! :wave:
+
+            Thank you for sending this PR!
+
+            We expect the following in all Pull Requests (PRs).
+            - PRs must be sent to the [appropriate branch](https://github.com/codeigniter4/CodeIgniter4/blob/develop/contributing/pull_request.md#branching)
+            - All git commits must be [GPG-signed](https://github.com/codeigniter4/CodeIgniter4/blob/develop/contributing/pull_request.md#signing)
+            - Must follow our [style guide](https://github.com/codeigniter4/CodeIgniter4/blob/develop/contributing/pull_request.md#php-style)
+            - Be [commented](https://github.com/codeigniter4/CodeIgniter4/blob/develop/contributing/pull_request.md#comments) in the PHP source file
+            - Be documented in the [user guide](https://github.com/codeigniter4/CodeIgniter4/blob/develop/contributing/pull_request.md#user-guide)
+            - Be [unit tested](https://github.com/codeigniter4/CodeIgniter4/blob/develop/contributing/pull_request.md#unit-testing)
+            - Pass all checks in GitHub Actions
+
+            > [!IMPORTANT]
+            > We expect all code changes or bug-fixes to be accompanied by one or more tests added to our test suite to prove the code works.
+
+            If pull requests do not comply with the above, they will likely be closed. Since we are a team of volunteers, we don't have any more time to work
+            on the framework than you do. Please make it as painless for your contributions to be included as possible.
+
+            See https://github.com/codeigniter4/CodeIgniter4/blob/develop/contributing/pull_request.md
+
+            Sincerely, the mergeable bot 🤖