docs(eslint-plugin): Add note for running eslint with ext for .ts fil…

…es (#547)

packages/eslint-plugin/README.md

@@ -68,6 +68,8 @@ If you want to use rules which require type information, you will need to specif
 
 See [@typescript-eslint/parser's README.md](../parser/README.md) for more information on the available "parserOptions".
 
+**Note: Make sure to use `eslint --ext .js,.ts` since by [default](https://eslint.org/docs/user-guide/command-line-interface#--ext) `eslint` will only search for .js files.**
+
 ## Usage with Prettier
 
 Install [`eslint-config-prettier`](https://github.com/prettier/eslint-config-prettier) to disable our code formatting related rules: