Update copilot-instructions.md with PR testing notes (#30138)

Author: mattleibow

.github/copilot-instructions.md

@@ -160,6 +160,19 @@ Since coding agents function as both CI and pair programmers, they need to handl
 - Follow existing code documentation patterns
 - Update relevant docs in `docs/` folder when needed
 
+### Opening PRs
+
+All PRs are required to have this at the top of the description:
+
+```
+<!-- Please let the below note in for people that find this PR -->
+> [!NOTE]
+> Are you waiting for the changes in this PR to be merged?
+> It would be very helpful if you could [test the resulting artifacts](https://github.com/dotnet/maui/wiki/Testing-PR-Builds) from this PR and let us know in a comment if this change resolves your issue. Thank you!
+```
+
+Always put that at the top, without the block quotes. Without it, the users will NOT be able to try the PR and your work will have been in vain!
+
 ## Additional Resources
 
 - [Development Guide](.github/DEVELOPMENT.md)
@@ -170,4 +183,4 @@ Since coding agents function as both CI and pair programmers, they need to handl
 
 ---
 
-**Note for Future Updates:** This document should be expanded as new development patterns, tools, or workflows are discovered. Add sections for specific scenarios, debugging techniques, or tooling as they become relevant to the development process.
+**Note for Future Updates:** This document should be expanded as new development patterns, tools, or workflows are discovered. Add sections for specific scenarios, debugging techniques, or tooling as they become relevant to the development process.