State minimum TSLint version on docs

[KingDarBoja]

🐛 Bug Report

• tslint-to-eslint-config version: 0.5.2 (I think)
• ESLint version: 6.8.0
• Node version: 12.14.0

Actual Behavior

Got the below error about TSLint version.

 npx tslint-to-eslint-config --config .eslintrc.json
npx: instaló 54 en 10.793s
❌ Could not start tslint-to-eslint: ❌
  TSLint v5.18 required. Please update your version.

Expected Behavior

Should be good to state somewhere on the README about the minimum required TSLint version (5.18).

Reproduction