docs: added defaultOptions

Author: talohana

README.md

@@ -44,28 +44,49 @@ empty lines) and replace it with the proper embed-code.
 <!-- START doctoc generated TOC please keep comment here to allow auto update -->
 <!-- DON'T EDIT THIS SECTION, INSTEAD RE-RUN doctoc TO UPDATE -->
 
+- [The problem](#the-problem)
+- [This solution](#this-solution)
+- [Table of Contents](#table-of-contents)
 - [Installation](#installation)
 - [Usage](#usage)
 - [Supported services](#supported-services)
   - [CodePen](#codepen)
+    - [Usage](#usage-1)
   - [CodeSandbox](#codesandbox)
+    - [Usage](#usage-2)
   - [GIPHY](#giphy)
+    - [Usage](#usage-3)
   - [Instagram](#instagram)
+    - [Usage](#usage-4)
   - [Lichess](#lichess)
+    - [Usage](#usage-5)
   - [Pinterest](#pinterest)
+    - [Usage](#usage-6)
   - [Slides](#slides)
+    - [Usage](#usage-7)
   - [SoundCloud](#soundcloud)
+    - [Usage](#usage-8)
   - [Spotify](#spotify)
+    - [Usage](#usage-9)
   - [Streamable](#streamable)
+    - [Usage](#usage-10)
   - [Twitch](#twitch)
+    - [Usage](#usage-11)
+    - [Options](#options)
+      - [parent](#parent)
+        - [Netlify](#netlify)
+        - [Vercel](#vercel)
   - [Twitter](#twitter)
+    - [Usage](#usage-12)
   - [YouTube](#youtube)
+    - [Usage](#usage-13)
 - [Options](#options-1)
   - [customTransformers](#customtransformers)
     - [Properties](#properties)
       - [`getHTML(url, options)`](#gethtmlurl-options)
       - [`name`](#name)
       - [`shouldTransform(url)`](#shouldtransformurl)
+      - [`defaultOptions`](#defaultoptions)
     - [Example transformer](#example-transformer)
   - [services](#services)
 - [Inspiration](#inspiration)
@@ -744,18 +765,27 @@ as the second argument to [`getHTML`](#gethtmlurl-options).
 The `shouldTransform` method should check if the given URL matches the one
 intended to transform. It should return a boolean value.
 
+##### `defaultOptions`
+
+The `defaultOptions` is the value that will be merged with provided options and
+passed to [`getHTML`](#gethtmlurl-options) as a second argument.
+
 #### Example transformer
 
 ```js
 // some-site-transformer.js
-const getHTML = (url) => `<iframe src="${url}"></iframe>`;
+const getHTML = (url, options) =>
+  `<iframe src="${url}" style="width${width}"></iframe>`;
 
 const name = 'someSite';
 
 const regex = /^https?:\/\/some-site\.com\//;
+
 const shouldTransform = (url) => regex.test(url);
 
-module.exports = { getHTML, name, shouldTransform };
+const defaultOptions = { width: '100%' };
+
+module.exports = { getHTML, name, shouldTransform, defaultOptions };
 ```
 
 ### services