Add some website additional meta information (#75)

Co-authored-by: Flavio Corpa <flavio.corpa@47deg.com>

docs/_config.yml

@@ -2,7 +2,7 @@ title: Mu-Haskell
 #-------------------------
 name: Mu-Haskell
 #-------------------------
-description: Lorem ipusm
+description: Mu is a purely functional library for building microservices.
 #-------------------------
 author: 47 Degrees
 keywords: functional-programming, monads, monad-transformers, functional-data-structure, swift, bow, fp-types, adt, free-monads, tagless-final, mtl, for-comprehension, category-theory

docs/_includes/_head-docs.html

@@ -5,7 +5,7 @@
     <meta name="description" content="{{site.description}}">
     <meta name="keywords" content="{{site.keywords}}">
 
-    <meta property="og:image" content="" />
+    <meta property="og:image" content="{{ 'img/poster.png' | relative_url }}" />
     <meta property="og:title" content="{{site.name}}" />
     <meta property="og:site_name" content="{{site.name}}" />
     <meta property="og:url" content="https://higherkindness.io/mu-haskell" />
@@ -17,7 +17,7 @@
     <meta name="twitter:card" content="summary_large_image">
     <meta name="twitter:site" content="@47deg">
     <meta name="twitter:creator" content="@47deg">
-    <meta name="twitter:image" content="" />
+    <meta name="twitter:image" content="{{ 'img/poster.png' | relative_url }}" />
 
     <script defer src="{{ '/js/docs.js' | relative_url }}"></script>