feat(doc): add README

README.md

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+# PHP wrapper for the [age command line encryption tool](https://github.com/FiloSottile/age)
+Encrypt and decrypt files with age from PHP. This library is only a wrapper for the the command line tool, it does not implement the C2CP age specification in PHP.
+
+```php
+$age = new FileEncryption("hello.txt");
+$age->encrypt("hello.txt.age", "hello.txt.age.key");
+```
+```php
+$age = new FileEncryption("hello.txt.age");
+$age->decrypt("hello.txt.age.key", "hello-decrypted.txt");
+```
+
+## Installation
+1. [Install the age command line tool](https://github.com/FiloSottile/age#installation)
+2. Install this library with composer
+   ```
+   composer require victorwesterlund/php-age
+   ```
+3. Import and use in your project
+
+## How to use
+Import and use the library:
+```php
+require_once "vendor/autoload.php";
+
+use \Age\FileEncryption;
+```
+
+### Encrypt a file
+Encrypt a file on disk by passing it to the `FileEncryption` constructor
+```php
+// Relative or absolute path to a file that should be encrypted
+$age = new FileEncryption("hello.txt");
+// Encrypted file destination and private key destination
+// This method also returns the private key as a string
+$age->encrypt("hello.txt.age", "hello.txt.age.key");
+```
+
+You can enable optional PEM encoding by chaining `armor()` before `encrypt()`
+```php
+// Encrypt with PEM encoding
+$age->armor()->encrypt("hello.txt.age", "hello.txt.age.key");
+```
+
+### Decrypt a file
+Decrypt a file on disk by passing it to the `FileEncryption` constructor
+```php
+// Relative or absolute path to a file that should be decrypted
+$age = new FileEncryption("hello.txt.age");
+// Private key file and destination of decrypted file
+$age->decrypt("hello.txt.age.key", "hello.txt");
+```