chore: Add changelogs section in PR template (#852)

### Changelogs
- Add changelogs section in PR template (internal)

Author: chohongm

.github/pull_request_template.md

@@ -23,6 +23,11 @@ Bullet points are also fine.
 // Recommended if you are dealing with SB_ISSUES (Customer issues) or release tickets.
 Fixes: [<TICKET_ID>](https://sendbird.atlassian.net/browse/<TICKET_ID>)
 
+// Section: Changelogs (Recommended)
+// Add (internal) at the end of each changelog if internal. 
+### Changelogs
+- 
+
 // If you want to add coauthors - for example, if you pair programmed or they made significant contributions to the ideas in the code and you want to thank them.
 Co-authored-by: Name <[email protected]>, Name2 <[email protected]>
 ```