add README

Author: deminy

README.md

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+[![Build Status](https://travis-ci.org/Crowdstar/background-processing.svg?branch=master)](https://travis-ci.org/Crowdstar/background-processing)
+
+This package allows to continue processing PHP after having HTTP response sent back to the client under PHP-FPM.
+
+PHP functions added by this package are executed after HTTP response sent back to the client but before PHP shutdown (
+before any registered shutdown function is called).
+
+# Limitations and Side Effects
+
+This package is for PHP-FPM only. Don't try to run it under CLI, PHP built-in web server, mod_php or FastCGI since it
+won't work.
+
+This package doesn't end PHP session before running background tasks. Because of this, it could affect HTTP requests that
+ use same session. Thus, this package should be used only for PHP applications where session is not used, e.g., stateless
+ micro-services.
+
+After sending HTTP response back to client side, background functions added continue to run and the PHP-FPM process is
+still running. To avoid side effects on your web server, please use this package accordingly. You may consider to use
+some worker instances or queue servers instead. When using this package, you may consider following suggestions to
+minimize side effects:
+
+* increase child processes in PHP-FPM.
+* increase maximum execution time for PHP-FPM.
+
+# Installation
+
+```bash
+composer require crowdstar/background-processing:@dev
+```
+
+# Examples
+
+## 
+
+```php
+<?php
+use CrowdStar\BackgroundProcessing\BackgroundProcessing;
+
+$sum  = 0;
+$file = sys_get_temp_dir() . DIRECTORY_SEPARATOR . 'background-processing.txt';
+
+// First background task added.
+BackgroundProcessing::add(
+// increase variable $sum by the sum of given numbers. In this example, final value of $sum will be 7 (1+2+4).
+    function (int ...$params) use (&$sum) {
+        $sum += array_sum($params);
+    },
+    1,
+    2,
+    4
+);
+// Second background task added and will be executed after the first one.
+BackgroundProcessing::add(
+    function () use (&$sum, $file) {
+        // Number 7 calculated from first task will be written to the file.
+        file_put_contents($file, $sum);
+    }
+);
+
+// Number 0 will be returned back to HTTP client.
+echo "Current sum value is {$sum}. Please check file {$file} in the web server; final sum value there should be 7.\n";
+
+// Send HTTP response back to the client first, then run the two background tasks added.
+BackgroundProcessing::run();
+
+// Anything here also runs in background.
+echo "This message won't shown up in HTTP response.";
+?>
+```