Adding installing and usage instructions to the readme file

Author: andergmartins

README.md

@@ -3,3 +3,129 @@
 This is a library for protecting WordPress plugins to run twice instances at the same time.
 
 ## Installation
+
+This library should be added as a composer requirement using the command:
+
+```shell
+composer require publishpress/publishpress-instance-protection
+```
+
+## How to use it
+
+### Making sure the plugin do not break with more running instances
+
+The first step, before even requiring this library, is making sure your plugin do not break if it has two or more instances
+running at the same time on the site.
+
+This is easy to accomplish using constants defined after the plugin is loaded, and checking it before loading the plugin.
+If the constant is defined, we do not load the plugin again (and not even require its libraries).
+
+This verification has to be out of any hook.
+
+Make sure to rename the variables and constant according to your plugin. This example copy the PublishPress Authors code:
+
+#### Free plugin example
+
+```php
+if (! defined('PP_AUTHORS_LOADED')) {
+    // include libraries, and dependencies
+    // instantiate the plugin, hooks, etc
+
+    define('PP_AUTHORS_LOADED', true);
+}
+```
+
+#### Pro plugin example
+
+```php
+if (! defined('PP_AUTHORS_PRO_LOADED') && ! defined('PP_AUTHORS_LOADED')) {
+    // include libraries, and dependencies
+    // instantiate the plugin, hooks, etc
+    // initialize the free plugin
+
+    define('PP_AUTHORS_PRO_LOADED', true);
+}
+```
+
+Please note that the Pro plugin checks two constants: its own constant, and the one defined in the free plugin. This
+way the Pro won't run if the free is already running as a stand alone plugin.
+
+### Adding the admin notices library
+
+This library do not use composer's autoloader because it needs to be loaded before everything else in the plugins. But
+it has its own autoloader inside it, following the PSR-4 pattern.
+
+You should always check if the vendor folder is on the expected path before trying to include anything from inside it.
+If not on the standard folder, make sure to give an option to the use to define a constant that gives the custom path of
+the vendor directory.
+
+*Check the following example on how to include and instantiate the library, just make sure to add this code as the
+first thing that will be executed in your plugin, on the root context, out of any hook.*
+
+#### Free Plugin example
+
+```php
+<?php
+$includeFilebRelativePath = '/publishpress/publishpress-instance-protection/include.php';
+if (file_exists(__DIR__ . '/vendor' . $includeFilebRelativePath)) {
+    require_once __DIR__ . '/vendor' . $includeFilebRelativePath;
+} else if (defined('PP_AUTHORS_VENDOR_PATH') && file_exists(PP_AUTHORS_VENDOR_PATH . $includeFilebRelativePath)) {
+    require_once PP_AUTHORS_VENDOR_PATH . $includeFilebRelativePath;
+}
+
+if (class_exists('PublishPressInstanceProtection\\Config')) {
+    $pluginCheckerConfig = new PublishPressInstanceProtection\Config();
+    $pluginCheckerConfig->pluginSlug = 'publishpress-authors';
+    $pluginCheckerConfig->pluginName = 'PublishPress Authors';
+
+    $pluginChecker = new PublishPressInstanceProtection\InstanceChecker($pluginCheckerConfig);
+}
+```
+
+### Pro Plugin example
+
+```php
+<?php
+$includeFilebRelativePath = '/publishpress/publishpress-instance-protection/include.php';
+if (file_exists(__DIR__ . '/vendor' . $includeFilebRelativePath)) {
+    require_once __DIR__ . '/vendor' . $includeFilebRelativePath;
+} else if (defined('PP_AUTHORS_VENDOR_PATH') && file_exists(PP_AUTHORS_VENDOR_PATH . $includeFilebRelativePath)) {
+    require_once PP_AUTHORS_VENDOR_PATH . $includeFilebRelativePath;
+}
+
+if (class_exists('PublishPressInstanceProtection\\Config')) {
+    $pluginCheckerConfig = new PublishPressInstanceProtection\Config();
+    $pluginCheckerConfig->pluginSlug = 'publishpress-authors-pro';
+    $pluginCheckerConfig->pluginName = 'PublishPress Authors Pro';
+    $pluginCheckerConfig->isProPlugin = true;
+    $pluginCheckerConfig->freePluginName = 'PublishPress Authors';
+
+    $pluginChecker = new PublishPressInstanceProtection\InstanceChecker($pluginCheckerConfig);
+}
+```
+
+The only required change is that you need to include two more configurations: `isProPlugin` and `freePluginName`.
+
+### Final example
+
+The final code should looks something like:
+
+```php
+$includeFilebRelativePath = '/publishpress/publishpress-instance-protection/include.php';
+if (file_exists(__DIR__ . '/vendor' . $includeFilebRelativePath)) {
+    require_once __DIR__ . '/vendor' . $includeFilebRelativePath;
+} else if (defined('PP_AUTHORS_VENDOR_PATH') && file_exists(PP_AUTHORS_VENDOR_PATH . $includeFilebRelativePath)) {
+    require_once PP_AUTHORS_VENDOR_PATH . $includeFilebRelativePath;
+}
+
+if (class_exists('PublishPressInstanceProtection\\Config')) {
+    // ....
+}
+
+if (! defined('PP_AUTHORS_LOADED')) {
+    // include libraries, and dependencies
+    // instantiate the plugin, hooks, etc
+
+    define('PP_AUTHORS_LOADED', true);
+}
+```