Add Copilot instructions for PR error checks

Author: djezzzl

.github/copilot-instructions.md

@@ -0,0 +1,10 @@
+# Copilot Instructions
+
+Every PR must have no errors from:
+
+```
+bundle exec rubocop
+bundle exec rspec spec/n1_loader_spec.rb
+bundle exec rspec spec/n1_loader_spec.rb spec/activerecord_spec.rb
+bundle exec rspec spec/n1_loader_spec.rb spec/activerecord_spec.rb spec/ar_lazy_preload_spec.rb
+```