added missing PT Serif font in docs section

Author: fritzvd

_layouts/docs.html

@@ -18,6 +18,10 @@
     <script src='https://cdn.rawgit.com/rstacruz/flatdoc/v0.9.0/theme-white/script.js'></script>
     <link rel="shortcut icon" href="/favicon.ico" type="image/x-icon" />
 
+    <!-- Fonts -->
+
+    <link href='https://fonts.googleapis.com/css?family=PT+Serif' rel='stylesheet' type='text/css'>
+
     <!-- Meta -->
     <meta content="yargs" property="og:title">
     <meta content="yargs the modern, pirate-themed successor to optimist." name="description">