chore(docs): add readme to jest-types (jestjs#12791)

packages/jest-types/README.md

@@ -0,0 +1,30 @@
+# @jest/types
+
+This package contains shared types of Jest's packages.
+
+If you are looking for types of [Jest globals](https://jestjs.io/docs/api), you can import them from `@jest/globals` package:
+
+```ts
+import {describe, expect, it} from '@jest/globals';
+
+describe('my tests', () => {
+  it('works', () => {
+    expect(1).toBe(1);
+  });
+});
+```
+
+If you prefer to omit imports, a similar result can be achieved installing the [@types/jest](https://npmjs.com/package/@types/jest) package. Note that this is a third party library maintained at [DefinitelyTyped](https://github.com/DefinitelyTyped/DefinitelyTyped/tree/master/types/jest) and may not cover the latest Jest features.
+
+Another use-case for `@types/jest` is a typed Jest config as those types are not provided by Jest out of the box:
+
+```ts
+// jest.config.ts
+import {Config} from '@jest/types';
+
+const config: Config.InitialOptions = {
+  // some typed config
+};
+
+export default config;
+```