chore: update issue template for rule suggestions

Author: adidahiya

.github/ISSUE_TEMPLATE/rule-suggestion.md

@@ -6,6 +6,8 @@ about: Suggest a new rule for TSLint
 
 ### Rule Suggestion
 
+_Note: TSLint has stopped accepting new core rules because it is being deprecated in favor of ESLint. Your request is likely to be rejected with the suggestion to maintain a custom rule outside this repository. See [the roadmap](https://github.com/palantir/tslint/issues/4534) for more details._
+
 **Is your rule for a general problem or is it specific to your development style?**
 
 **What does your suggested rule do?**