docs: add deprecation notice (#4935)

Author: orta

docs/index.md

@@ -4,6 +4,8 @@ layout: default
 subtitle: An extensible linter for the TypeScript language.
 ---
 
+:warning: __TSLint [has been deprecated](https://medium.com/palantir/tslint-in-2019-1a144c2317a9) as of 2019__. Please see this issue for more details: [Roadmap: TSLint → ESLint](https://github.com/palantir/tslint/issues/4534). If you're interested in helping with the TSLint/ESLint migration, please check out our [OSS Fellowship](https://medium.com/palantir/fellowships-for-open-source-developers-3892e6b75ee1) program.
+
 TSLint is an extensible static analysis tool that checks [TypeScript][0] code for readability, maintainability, and functionality errors. It is widely supported across modern editors & build systems and can be customized with your own lint rules, configurations, and formatters.
 
 ## Quick start