docs: add note for typescript users.

README.md

@@ -154,7 +154,23 @@ client
 
 ### Nodemailer Transport
 
-***[Nodemailer](https://www.npmjs.com/package/nodemailer) needed as a dependency.***
+> NOTE: [Nodemailer](https://www.npmjs.com/package/nodemailer) is needed as a dependency.
+
+```sh
+yarn add nodemailer
+
+# or, if you are using NPM:
+npm install --s nodemailer
+```
+
+If you're using Typescript, install `@types/nodemailer` as a `devDependency`.
+
+```sh
+yarn add --dev @types/nodemailer
+
+# or, if you are using NPM:
+npm install --s-dev @types/nodemailer
+```
 
 You can provide Mailtrap specific keys like `category`, `customVariables`, `templateUuid` and `templateVariables`.