Add metatags for sharing

[andydemeulemeester]

<meta property="og:title" content="Example Page">
<meta property="og:image" content="https://example.com/images/image.jpg">
<meta property="og:description" content="This is just an example page.">
<meta property="og:url" content="https://example.com/page.html">
<meta name="twitter:card" content="summary_large_image">

Simple stuff like thorblock's logo, pool name, pool minimum amount required and total size. This way it is easier to see what people are actually linking.