Merge pull request #12537 from pawelskowronek/patch-1

Add missing instruction if you are adding typescript

Author: matrush

docusaurus/docs/adding-typescript.md

@@ -37,7 +37,9 @@ or
 yarn add typescript @types/node @types/react @types/react-dom @types/jest
 ```
 
-Next, rename any file to be a TypeScript file (e.g. `src/index.js` to `src/index.tsx`) and **restart your development server**!
+Next, rename any file to be a TypeScript file (e.g. `src/index.js` to `src/index.tsx`) and create tsconfig.json if it's not in the root of your project [`tsconfig.json` file](https://www.typescriptlang.org/docs/handbook/tsconfig-json.html).
+
+Finally **restart your development server**!
 
 Type errors will show up in the same console as the build one. You'll have to fix these type errors before you continue development or build your project. For advanced configuration, [see here](advanced-configuration.md).