Update docs changelog (for PR #57007) (#57084)

Co-authored-by: github-actions[bot] <github-actions[bot]@users.noreply.github.com>
Co-authored-by: Sophie <29382425+sophietheking@users.noreply.github.com>

Author: 3 people

CHANGELOG.md

@@ -2,6 +2,10 @@
 
 **12 August 2025**
 
+We’ve updated the documentation for Copilot repository custom instructions to go with the release that now brings this feature to the Eclipse IDE.
+
+See: [Adding repository custom instructions for GitHub Copilot](https://docs.github.com/en/copilot/how-tos/configure-custom-instructions/add-repository-instructions?tool=eclipse) and [About customizing GitHub Copilot Chat responses](https://docs.github.com/en/copilot/concepts/response-customization?tool=eclipse).
+
 We have added a tutorial for using Copilot to create Mermaid diagrams at [Creating Diagrams](https://docs.github.com/en/copilot/tutorials/copilot-chat-cookbook/communicate-effectively/creating-diagrams).
 
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