Add link to open and closed issues search to issue template

ISSUE_TEMPLATE.md

@@ -1,5 +1,5 @@
 **Godot version:**
-<!-- If thirdparty of self-compiled, specify the build date or commit hash. -->
+<!-- If thirdparty or self-compiled, specify the build date or commit hash. -->
 
 
 **OS/device including version:**
@@ -15,3 +15,7 @@
 
 **Minimal reproduction project:**
 <!-- Optional but greatly speeds up debugging. You can drag and drop a zip archive to upload it. -->
+
+
+- [ ] I searched the existing [GitHub issues](https://github.com/godotengine/godot/issues?utf8=%E2%9C%93&q=is%3Aissue+)
+  for potential duplicates.