The Pronamic Manuals plugin allows you to easily add the GitHub manuals to your WordPress website.

composer.json

{
    "name": "remco/pronamic-manuals",
    "authors": [
        {
            "name": "Remco Tolsma",
            "email": "[email protected]"
        }
    ],
    "require": {
        "michelf/php-markdown": "^1.6"
    }
}

pronamic-manuals.php

<?php
/*
Plugin Name: Pronamic Manuals
Plugin URI: http://www.pronamic.eu/plugins/pronamic-manuals/
Description: The Pronamic Manuals plugin allows you to easily add the GitHub manuals to your WordPress website.

Version: 1.0.0
Requires at least: 3.6

Author: Pronamic
Author URI: http://www.pronamic.eu/

Text Domain: pronamic-manuals
Domain Path: /languages/

License: GPL

GitHub URI: https://github.com/pronamic/wp-pronamic-manuals
*/

/**
 * Autoload
 */
require_once plugin_dir_path( __FILE__ ) . 'vendor/autoload.php';

/**
 * Shortcode
 *
 * @see https://michelf.ca/projects/php-markdown/configuration/
 */
function shortcode_manual() {
	$content = '';

	$url = 'https://raw.githubusercontent.com/wp-pay/test-suite/master/manuals/3.9.0/formidable/2.0.22/buckaroo/README.md';
	$dir = dirname( $url );

	$result = wp_remote_get( $url );

	if ( 200 === wp_remote_retrieve_response_code( $result ) ) {
		$content = wp_remote_retrieve_body( $result );

		$parser = new \Michelf\Markdown();

		$parser->header_id_func = function ( $header ) {
			return sanitize_title( $header );
		};

		$parser->url_filter_func = function( $url ) use ( $dir ) {
			if ( '#' === $url[0] ) {
				return $url;
			}

			return $dir . '/' . $url;
		};

		$content = $parser->transform( $content );
	}

	return $content;
}

add_shortcode( 'manual', 'shortcode_manual' );