add link to available Typography.js themes (#2355)

docs/tutorial/part-two/index.md

@@ -219,7 +219,7 @@ Stop `gatsby develop` and then restart it again for our plugin change to take ef
 
 Now all the text font sizes should be slightly bigger. Try changing the `baseFontSize` to `24px` then `12px`. All elements get resized as their `font-size` is based on the `baseFontSize`.
 
-There are many themes available for Typography.js. Let's try a couple. Run in your terminal at the root of your site:
+There are [many themes available](https://github.com/KyleAMathews/typography.js#published-typographyjs-themes) for Typography.js. Let's try a couple. Run in your terminal at the root of your site:
 
 ```shell
 npm install --save typography-theme-bootstrap typography-theme-lawton