README: Add install info.

README.md

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+Invoice generator in PHP
+========================
+
+Smart invoice generator to PDF.
+
+📦 Installation
+---------------
+
+It's best to use [Composer](https://getcomposer.org) for installation, and you can also find the package on
+[Packagist](https://packagist.org/packages/baraja-core/invoice) and
+[GitHub](https://github.com/baraja-core/invoice).
+
+To install, simply use the command:
+
+```shell
+$ composer require baraja-core/invoice
+```
+
+You can use the package manually by creating an instance of the internal classes, or register a DIC extension to link the services directly to the Nette Framework.
+
+📄 License
+-----------
+
+`baraja-core/invoice` is licensed under the MIT license. See the [LICENSE](https://github.com/baraja-core/template/blob/master/LICENSE) file for more details.