chore: add simple readme description

Signed-off-by: Bart Jaskulski <bartek.jaskulski@wpdesk.net>

README.md

+# WP Migrations
+
+Simple library based on `doctrine/migrations` for managing database
+versioned schema.
+
+## Installation
+
+`composer require wpdesk/wp-migrations`
+
+## Usage
+
+To operate on WordPress database you need to create class extending
+`WPDesk\Migrations\AbstractMigration`.
+
+```php
+$migrator = WpdbMigrator::from_classes(
+  [Version_01012022::class, Version_12012022::class],
+  'unique_db_option_name'
+);
+$migrator->migrate();
+```
+
+With multiple migration sources it may be inconvenient to type each class in array. In such case it may be better to group all migrations in directories. In such case it is required that (besides each class MUST extend `WPDesk\Migrations\AbstractMigration` class) file names starting with **Version** prefix.
+
+Assuming following directory structure:
+
+```
+project\
+  migrations\
+    Version_01012022.php
+    Version_12012022.php
+    XVersion_13012022.php # This will be skipped
+  main.php
+```
+
+You can register your migration classes in the following way:
+
+```php
+$migrator = WpdbMigrator::from_directories(
+  [__DIR__ . '/migrations'],
+  'unique_db_option_name'
+);
+$migrator->migrate();
+```