Update change_request.md

Improve description so responses on the tickets are less repetitive.

.github/ISSUE_TEMPLATE/change_request.md

@@ -7,7 +7,8 @@ assignees: ''
 
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 **Which expansion shall be added?**
-Give the the download url.
+Give the the download url for the expansion.
+Give the url to the thread discussing the new expansion on https://bb.oolite.space. That thread needs to tell everyone the name of the oxp, what to expect, where they can download, and that you’re looking for testers. That thread will then become the main place to discuss your mod going forward.
 
 **Which expansion shall be removed?**
 Give the expansion id and version, or the download url.