Add README to present the library and show how to use it.

README.md

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+# Forwarded
+
+Forwarded is a PHP library which parse the `Forwarded` request header as defined in [RFC 7239](https://datatracker.ietf.org/doc/html/rfc7239). The `Forwarded` header is a standardized method for identifying the originating IP address of a client connecting to a web server through an HTTP proxy or load balancer, for example.
+
+Forwarded is distributed as a composer library.
+
+## Getting started
+
+Just add the dependency to your composer project:
+
+```shell
+composer require madeorsk/forwarded
+```
+
+## How to use
+
+
+### The easy way
+
+Usually, you just want to get the Forwarded header from your current request (for example when using PHP-FPM), or even just the IP address of the origin of the request.
+
+```php
+<?php
+
+use Madeorsk\Forwarded\Forwarded;
+
+$forwarded = Forwarded::get(); // Getting the parsed Forwarded header.
+
+$originIp = Forwarded::getOriginIp(); // Getting the IP address of the origin request from the parsed Forwarded header.
+
+```
+
+### If you need more control
+
+If you cannot obtain the header content from the classic standard way, you can call directly the Forwarded header parse.
+
+```php
+<?php
+
+use Madeorsk\Forwarded\Parser;
+
+// Get a Forwarded parsed header from its raw string.
+$forwarded = (new Parser())->parse("for=192.0.2.43,for=\"[2001:db8:cafe::17]\",for=unknown");
+
+// Get the Forwarded header content parsed in a raw associative array from its raw string.
+$forwardedAssoc = (new Parser())->parseAssoc("for=192.0.2.43,for=\"[2001:db8:cafe::17]\",for=unknown");
+
+```