.github: Add repo lockdown GitHub workflow and update .gitignore

This commit adds a new GitHub Actions workflow for repository
lockdown. The workflow triggers when a pull request is opened, and it
notifies the contributor to use the OpenSBI mailing list for patch
reviews.

Additionally, the .gitignore file is updated to include the .github
directory.

Signed-off-by: Dongdong Zhang <zhangdongdong@eswincomputing.com>
Reviewed-by: Anup Patel <anup@branifault.org>

.github/workflows/repo-lockdown.yml

@@ -0,0 +1,26 @@
+name: 'Repo Lockdown'
+
+on:
+  pull_request_target:
+    types: opened
+
+permissions:
+  pull-requests: write
+
+jobs:
+  action:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: dessant/repo-lockdown@v4
+        with:
+          pr-comment: |
+            We have mailing list based patch review so it would be great if you can send these patchs to OpenSBI mailing list.
+
+            You need to join OpenSBI mailing list using following link
+            http://lists.infradead.org/mailman/listinfo/opensbi
+
+            Make sure you use "git send-email" to send the patches.
+
+            Thanks for your contribution to OpenSBI project.
+          lock-pr: true
+          close-pr: true

.gitignore

@@ -4,6 +4,7 @@
 # exceptions we need even begin with dot
 !.clang-format
 !.gitignore
+!.github
 
 # Object files
 *.o