Mention Node 12 LTS requirement in README.md

Fixes #260

Author: Josh Goldberg

README.md

@@ -32,6 +32,8 @@ The `tslint-to-eslint-config` command reads in any existing linter, TypeScript,
 For any TSLint rules with corresponding ESLint equivalents, those equivalents will be used in the new configuration.
 TSLint rules without ESLint equivalents will be wrapped with [eslint-plugin-tslint](https://github.com/typescript-eslint/typescript-eslint/tree/master/packages/eslint-plugin-tslint).
 
+> Requires Node 12+ (LTS)
+
 ### CLI Flags
 
 Each of these flags is optional: