Update border documentation

docs/input/appendix/borders.md

@@ -0,0 +1,19 @@
+Title: Borders
+Order: 2
+---
+
+There is different built-in borders you can use for tables and panels.
+
+# Built-in borders
+
+<img src="../assets/images/borders.png" style="max-width: 100%;">
+
+# Usage
+
+To create a table and set it's border to `SimpleHeavy` as seen in the 
+image above:
+
+```csharp
+var table = new Table();
+table.Border = Border.SimpleHeavy;
+```