add metadescription to site

provides a description for screen readers and also a nice SEO boost.

_layouts/default.html

@@ -4,6 +4,7 @@
     <meta charset="UTF-8">
     <meta http-equiv="X-UA-Compatible" content="IE=edge">
     <meta name="viewport" content="width=device-width, initial-scale=1">
+    <meta name="description" content="homepage of the Exclamation Foundation, which is the organizing body behind !!Con.">
     <link href="https://fonts.googleapis.com/css?family=Lato:400,700,900&display=swap" rel="stylesheet">
     <link rel="stylesheet" href="{{ '/assets/css/style.css?v=' | append: site.github.build_revision | relative_url }}">
     <script src="{{ '/assets/script.js?v=' | append: site.github.build_revision | relative_url }}" defer></script>