Add github lockdown app to auto-close pull requests.

Author: tstellar

.github/lockdown.yml

@@ -0,0 +1,33 @@
+# Configuration for Repo Lockdown - https://github.com/dessant/repo-lockdown
+
+# Skip issues and pull requests created before a given timestamp. Timestamp must
+# follow ISO 8601 (`YYYY-MM-DD`). Set to `false` to disable
+skipCreatedBefore: false
+
+# Issues and pull requests with these labels will be ignored. Set to `[]` to disable
+exemptLabels: []
+
+# Comment to post before closing or locking. Set to `false` to disable
+comment: true
+
+# Label to add before closing or locking. Set to `false` to disable
+label: false
+
+# Close issues and pull requests
+close: true
+
+# Lock issues and pull requests
+lock: true
+
+# Limit to only `issues` or `pulls`
+# only: issues
+
+# Optionally, specify configuration settings just for `issues` or `pulls`
+# issues:
+#   label: wontfix
+
+pulls:
+  comment: >
+    This repository does not accept pull requests.
+    Please follow http://llvm.org/docs/Contributing.html#how-to-submit-a-patch for contribution to LLVM. 
+