update the issue templates for package:lints (#887)

Author: devoncarew

.github/ISSUE_TEMPLATE/lints.md

@@ -3,3 +3,11 @@ name: "package:lints"
 about: "Create a bug or file a feature request against package:lints."
 labels: "package:lints"
 ---
+
+**Note** this issue tracker is used to:
+
+- suggest adding an _existing_ lint to the core or recommended lint sets, or
+- file a bug or enhancement against `package:lints`
+
+To propose an entirely new lint, see instead https://github.com/dart-lang/sdk;
+thanks!

.github/ISSUE_TEMPLATE/lints_propoposal.md

@@ -110,6 +110,6 @@ for a list of rules that make up the core and recommended rule sets.
 [customizing static analysis]: https://dart.dev/tools/analysis
 [rules]: https://dart.dev/tools/linter-rules
 [pub.dev]: https://pub.dev
-[issue]: https://github.com/dart-lang/core/issues
+[issue]: https://github.com/dart-lang/core/issues/new?template=lints.md
 [sdk repo]: https://github.com/dart-lang/sdk
 [`package:flutter_lints`]: https://pub.dev/packages/flutter_lints