Add examples in readme (#8)

Extend readme with preview image and examples to code sandboxes

Author: igeligel

README.md

@@ -4,6 +4,11 @@
 
 Use this component to add a github gist on your website.
 
+<p><details>
+  <summary><b>Preview</b></summary>
+  <p><img src="https://i.imgur.com/4EYDskB.png" /></p>
+</details></p>
+
 Get the id from the gist url `https://gist.github.com/{your_name}/{id}` and set it as a property of the component.
 
 ## Example
@@ -20,6 +25,8 @@ React.render(
 );
 ```
 
+[![Edit react-gist-example-single-file](https://codesandbox.io/static/img/play-codesandbox.svg)](https://codesandbox.io/s/reactgistexamplesinglefile-z9vkc?fontsize=14)
+
 Multi-file gist:
 
 ```js
@@ -32,6 +39,8 @@ React.render(
 );
 ```
 
+[![Edit react-gist-example-multi-file](https://codesandbox.io/static/img/play-codesandbox.svg)](https://codesandbox.io/s/reactgistexamplemultifile-9rw4g?fontsize=14)
+
 ## Usage
 
 ### `<Gist id={string} file={string} />`