[FormatInterface] Add page

FormatInterface.md

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+The `FormatInterface`interface defines functions that need to be implemented by all formats:
+
+* [setItems](#the-setitems-function)
+* [display](#the-display-function)
+* [stringify](#the-stringify-function)
+
+Find a [template](#template) at the end of this file
+
+#Functions
+
+##The `setItems` function
+
+The `setItems` function receives an array of items generated by the bridge and must return the object instance. Each item represents an entry in the feed. For more information refer to [BridgeAbstract](BridgeAbstract#items).
+
+```PHP
+setItems(array $items): self
+```
+
+##The `display` function
+
+The `display` function shows the contents to the user and must return the object instance.
+
+```PHP
+display(): self
+```
+
+##The `stringify` function
+
+The `stringify` function returns the items received by [`setItems`](#the-setitem-function) as string.
+
+```PHP
+stringify(): string
+```
+
+#Template
+
+This is a bare minimum template for a format:
+
+```PHP
+<?php
+class MyTypeFormat implements FormatInterface {
+    public function setItems(array $items){
+        // Implement your code here!
+        return $this;
+    }
+
+    public function stringify(){
+        // Implement your code here
+        return '';
+    }
+
+    public function display(){
+        // Implement your code here
+        echo $this->stringify();
+        return $this;
+    }
+}
+// Imaginary empty line!
+```