Add .sr (screen reader) styling to CSS

Makes element non-visible, but still readible to screen-readers,
and passes accessibility checks (such as contrast, and size errors)

main.css

@@ -40,6 +40,11 @@ border:1px solid #DDD;border-radius:6px}
 float:right;
 padding:14px;border:1px solid;
 margin:1.4rem;margin-right:auto}
+.sr{
+position:absolute;overflow:hidden;
+left:-9999px;top:auto;
+width:1px;height:1px;
+}
 footer p{margin:0}
 
 table{text-align:left;width:100%;