docs: nominate-committer add announcement template (apache#4954)

website/community/pmc_members/nominate-committer.md

@@ -148,3 +148,19 @@ ${your_name} (as represents of The Apache OpenDAL PMC)
 After the candidate accepts the invitation and the iCLA is recorded, add the candidate to the committer list by [whimsy roster tools](https://whimsy.apache.org/roster/committee/opendal)
 
 ![](roster-add-committer.png)
+
+## Announcement
+
+Once the nominee accepts the invitation and the commit bit is granted, it's encouraged to send an announcement email to [email protected] to welcome the new committers. Here is an email template:
+
+```text
+Hello, everyone
+
+On behalf of the Apache OpenDAL PMC, I'm happy to announce that
+${candidate_name} has accepted the invitation to become a committer of
+Apache OpenDAL.
+
+Welcome, and thank you for your contributions!
+
+${your name}
+```