Add font-family-mono core token

Author: aweell

guidelines/principles/typography/README.md

@@ -16,9 +16,11 @@ Make sure you include all the different typographic variants in order to enhance
 
 For our sans-serif `font-family`, we use the humanist typeface Open Sans, designed by Steve Matteson and licensed under the [Apache License, Version 2.0.](https://www.apache.org/licenses/LICENSE-2.0.html)
 
-| Token                         | Description                                   | Property            | Value                        | 
-| :---                          | :---                                          | :---                | :---                         |  
-| `font-family-sans`            | _Default font family sans-serif_              | `font-family`       | 'Open Sans', sans-serif      | 
+| Token                         | Description                                   | Property            | Value                          | 
+| :---                          | :---                                          | :---                | :---                           |  
+| `font-family-sans`            | _Default font family sans-serif_              | `font-family`       | 'Open Sans', sans-serif        | 
+| `font-family-mono`            | _Default font family monospaced_              | `font-family`       |  Source Code Pro', monospace   | 
+
 
 ## Weight