enable GitHub Pages for hosting attachment files. (#139)

diff --git a/.asf.yaml b/.asf.yaml
index e8b8526..8a33587 100644
--- a/.asf.yaml
+++ b/.asf.yaml
@@ -16,6 +16,7 @@
     issues: true
     projects: false
 
+  ghp_branch:  gh-pages  # for attachments
 
 notifications:
   commits:      commits@lucene.apache.org