feat: Add a TypeScript usage section to the README (#827)

* Add a TypeScript usage section to the README

Additional examples should be added, such as how to type a page that requires additional props beside the t function.

* feat: Add TS doc with explanation on prop-types

Remove `prop-types` from TS example.

* feat: Move TS doc to separate file

* feat: Restore README formatting

README.md

@@ -219,6 +219,10 @@ _This table contains options which are specific to next-i18next. All other [i18n
 - [Localising `next/head` requires special consideration due to NextJs internals](https://github.com/isaachinman/next-i18next/issues/251#issuecomment-479421852).
 - [How to use multiple namespaces in the same component](https://github.com/isaachinman/next-i18next/issues/762#issuecomment-661348457)
 
+## Usage with TypeScript
+
+`next-i18next` is written in TypeScript and has full support for it. Refer to the usage guide [here](./TYPESCRIPT.md).
+
 ## Contributors
 
 Thanks goes to these wonderful people ([emoji key](https://github.com/kentcdodds/all-contributors#emoji-key)):

TYPESCRIPT.md

@@ -0,0 +1,41 @@
+# Usage with TypeScript
+
+`next-i18next` has full support for TypeScript. Types are included with the `next-i18next` package.
+
+## Usage with components
+
+To type the [t function](https://www.i18next.com/overview/api), use the `TFunction` type provided by `next-i18next`:
+
+```tsx
+// this is the same TFunction from 'i18next'
+import { TFunction } from 'next-i18next';
+
+const Heading = ({ t }: { readonly t: TFunction }) => <h1>{t('heading')}</h1>;
+```
+
+## Usage with pages
+
+Typing a [page](https://nextjs.org/docs/basic-features/pages) with the t function as the singular prop is similar to JS, but with TS instead of `prop-types`.
+TypeScript validates at _compile time_ while [`prop-types`](https://github.com/facebook/prop-types) validates at _runtime_.
+Although `prop-types` can do things that TS can't like value validation, TS alone will suffice in most cases.
+
+```tsx
+import { TFunction } from 'next-i18next';
+import { withTranslation } from '../i18n';
+
+const Page = ({ t }: { readonly t: TFunction }) => (
+  <div>
+    <h1>{t('heading')}</h1>
+  </div>
+);
+
+Page.getInitialProps = async () => ({
+  namespacesRequired: ['common'],
+});
+
+export default withTranslation('common')(Page);
+```
+
+## More
+
+The complete list of exported types can be found [here](./types.d.ts).